Water-based finish (KTM)


With personal health and environmental concerns (and regulations) ever increasing, the manufacturers of finishing materials have seen that the long term trend is moving away from solvent based finishes and toward less hazardous, less toxic products. In response, many manufacturers set the goal to manufacture a finish with all of the positive characteristics of current solvent based lacquers but without the hazardous, toxic solvent base.

There have been steady improvements in water based products intended to eventually replace the solvent based products for all applications. There have been hits and misses, but the currently available water based products are finally close enough to this goal to be used for guitar finishing. These products are now as easy to apply and work with as solvent based lacquers and give great results - especially considering the obvious advantages of using a less toxic water based system. Guitar builders using these products claim that they are achieving finishes that are comparable with those of the solvent based product.
As with the solvent based lacquer system, there are many good products available for pore filling, color shading, sanding, and buffing that are compatible with this water based urethane. Although most builders are spraying the water based urethane, this product also works well as a brush-on finish.
The advantages of using this water based urethanes are tremendous for all woodworkers. The health risks and environmental harm are diminished to almost nothing. Cleanup is easy with warm water. The vapors are not harmful or toxic so no breathing protection or strong ventilation is required. The water based urethane has very high solid content so less material is used to finish each instrument, and there is no need for lacquer thinner during spraying or cleanup. Any required thinning is accomplished with distilled water or a compatible reducer. This makes the water based system far more economical (guitars per gallon) than using solvent based lacquer. Because of its less toxic nature it can also be shipped without restriction - unlike hazardous/toxic materials.
We presently offer the KTM9 water based finish and are confident that it is a great water based finish to compete with nitrocellulose lacquers. It is clear, hard, repairable, economical, does not require a cross-linker or other additives, and shows no blue tint on the finished instrument. It is the finish that we recommend most highly to beginning and professional luthiers.

Other materials used with water-based lacquer:
• Staining raw wood  - Use either our Alcohol Soluble Aniline Dyes, FSB or Water Soluble Aniline Dyes, FSBW or dye concentrate, FMDY (Water-Soluble or MEK-Soluble).
• Shading color mixed into finish - Use either Water Soluble Aniline Dyes, FSBW or Water Soluble dye concentrate , FMDYWF.
• Pore filler - LMI Pore Filler works fine but for best results, use our System 3 clear epoxy filler ( FSB1.5K or FCC1.5K) with the silica thickener (FST1Q)
• Sealer –Use the finish itself as a sealer or one of our shellacs.

Some Frequently Asked KTM9 Questions

Are there instructions for how to apply this finish?
Yes, see articles below.

Does the KTM9 require a cross-linker?
No, this was only required on the previous KTM product, the KTM4

Does the KTM9 require a sealer?
No, you can use the finish itself as a sealer

How does it differ from previous water-based finishes?
The KTM9 is by far the best waterbased finish available. It is relatively problem free, is clearer than other products on the market and is harder.

Most Waterbased finishes have a blue cast when you turn the instrument in the light. Is this true for the KTM9?
No! The clarity of the KTM9 is substantially clearer than other waterbased finishes on the market.

How does it differ from Nitrocellulose lacquer?
Only the most experienced builders will notice a difference visually (assuming the finish was applied correctly). It is very clear and transparent and can be applied thin so as not to inhibit the tone of the instrument. It is much harder than any other waterbased lacqer, and almost as hard as Nitrocellulose. It is similarly easy to repair.

One great advantage over lacquer that the KTM9 has is that it is much, much safer to use. You do not need to buy a spray booth to apply it and it can be shipped anywhere. It is also better for your health (lacquer can cause respiratory and other problems with prolonged inhalation or exposure) and is much kinder to the environment as well.

I have applied the finish and am having trouble achieving a flat surface. What did I do wrong?
Achieving a good finish is always challenging but KTM9 is relatively easy to use and immune to many common problems. Still, finish work is never simple and you must take care to make sure the finish comes out perfect. If you are having troubles chances are that the finish is not at fault! The most common culprits are:

Can I apply the KTM9 over an oil finish, shellac or a solvent based sealer?
KTM9 adheres well to shellac, but it is probably best to avoid applying it over oil, varnish, or other solvent based products such as nitrocellulose lacquer or sealer. The only products that we know for sure to be safe are flake shellac (like we carry, avoid the pre-mixed hardware store variety) our Microbead filler, the System 3 epoxy filler and the previous KTM product, KTM4. It's possible that it may be compatible with other waterbased products available, but we can not be sure.

Do I need to use a spray booth when finishing with KTM9?
A spray booth helps with ventilation (which is absolutely essential with a nitrocellulose finish) and helps to eliminate dust in the air. Since ventilation is not so critical with the waterbased finish (you can finish next to a fan near an open window) you can do without a spray booth if you have adequate dust control in your finishing area.

Can I brush on the KTM9 finish?
Spraying this (or any other) finish is best because the coats can be applied thinly and evenly –but you can use a brush with excellent results.

How do I get the best results when using a brush?
Use a fine cell foam brush (available at any paint or hardware store). Use the flat portion of the brush near the edge and avoid using the point of the brush. Do not over-saturate the brush with finish. Practice applying the finish as thin and even as possible before applying the finish to your instrument. As with any finish, it is best to work in a dry, humidity controlled environment.

How many instruments can I finish with a quart of KTM9?
Approximately 4 to 6 instruments. Compared with Nitrocellulose lacquer it is a very economical product.

Are there compatibility issues with other products?
We have heard that it is best to use our FGX yellow glue since regular Titebond tends to cause sinking around binding and purfling joints. It can accept our Waterbased Metal Acid dyes and our Water based Aniline dyes. If you are dying the wood directly (as opposed to adding it to the finish) be sure to use the alcohol or MEK based dyes to keep the colors from running.

Are there shipping restrictions with this product?
Unlike Nitrocellulose lacquers, the KTM9 can be shipped overseas or by air delivery.

What pore fillers can I use?
The clear System 3 epoxy filler we carry will give you the best results. It enhances the appearance of depth and the durability of the resulting finish. Our microbead fillers work well if you must use a colored filler.

Can it be buffed or polished?
Yes, it can be machine buffed or hand polished just like Nitrocellulose finishes.


KTM-9 successfully creates a nitro-lacquer like finish, but with the environmental and health advantages of water based chemistry. It is simple and safe to use, very forgiving in the application process, has all of the desirable physical and visual properties of the solvent base finishes, and saves time. KTM-9 is a self-cross-linking urethane/acrylic co-polymer which cures to hard in one week. The application process takes two days, spraying 4-6 coats per day and sanding lightly between every other coat (as necessary). Because the flow out is so good, there is very little leveling necessary and runs are rare, even with heavy application. Here is John Greven's method of applying KTM-9 which maximizes its lacquer-like look and durability.




The resulting finish will dazzle. It is totally transparent, warm and crystalline like nitrocellulose lacquer. It looks deep without being thick. The film is tough and hard as well as very resistant to solvents and sweat. Repairs are easy as well. A light sanding and recoating will bring back the finish to original perfection.

One last note: the KTM-9 is backward compatible with other water base finishes should you need to overtop an older film with the new material. Light sanding of the old finish should allow for minimal witness lines and excellent adhesion.

Following are two articles on finishing with KTM-9 water-based lacquer by Keith Rhodes and Mike Doolin

by Keith Rhodes

KTM-9 is a waterborne finish which creates a finish similar to a nitro-cellulose lacquer finish. It is a self-cross-linking urethane acrylic co-polymer. Applying this finish can be completed in approximately one week. The finish is applied directly from the can with no thinning required. It has good flow characteristics, so that not much sanding is required between coats. The final buffed finish is extremely clear.

I have built classical guitars for several years, always using a hand applied French polish finish to the entire guitar. (The results have always been pleasing but I have often been disappointed when people have played my guitars and have returned them with scratches-a result of somewhat careless handling and the softness of the French polish.) With this in mind, I decided to apply KTM-9, a hard wearing finish to the back and sides of my instruments. All of my soundboards for the present time, however, will still be hand finished with shellac.

Having a small workshop and not wishing to purchase expensive spraying equipment or build a spray booth, I decided to try hand brushing the finish coats of KTM-9. I read both Mike Doolin’s and John Greven’s articles about spray finishing with KTM-9. I found the articles to be well written and very informative and I followed many of their recommendations. I was not able, however, to track down any information in these two articles or indeed anywhere about hand brushing this finish, so I basically used a method similar to applying a brushed traditional violin varnish finish and followed my intuition.

Here is the way I hand brushed and finished a classical guitar. I hope that you will find the description useful for finishing your own instruments.

I finish sanded the instrument as I normally do. This particular wood required a pore filler, so I applied two thin coats of Z-poxy. Working with a small area at a time, I used an angled credit card to force the epoxy into the pores. The card was then used to remove as much of the still liquid epoxy as possible. A safety razor works equally well for this task. The idea is to apply very thin and even coats of epoxy. A strong back light is essential to judge what is happening with the epoxy. Backlighting is also important later on, both when working with the KTM-9 and during the buffing operation.

The epoxy coats were applied 24 hours apart, sanding between coats using 320 grit wet/dry paper. As you would with any sanding operation, be sure to use a good quality fine particle filtering face mask when sanding the dried epoxy. Water was used as a lubricant, as it keeps the dust down and creates a slurry which speeds the cutting action. It is important not to sand through the epoxy to bare wood because this will show through the final finish. In most cases, after two coats of epoxy, I ended up with a flat, even surface free of pores. If I was not happy with the result, I would continue to apply more coats of epoxy, following the same 24 hour time schedule.

A varnish brush is not suitable for KTM-9 finish because the bristle soaks up water and the finish is water based. For about $4, I bought a 2" polyester “Shortcut” brush by Wooster Co. at Home Depot. The brush is easy to handle and has splayed bristle ends which help the finish coats to flow well. This brush does not absorb water.

I began by heating the can of KTM-9 by placing it in a large pan of hot water for 10 minutes, which I think helped the finish to flow from the brush more easily. While this was cooking, I spread clean paper on the work bench and gathered some paper towels.

To charge the dry brush, I dipped lower quarter of the bristles into the finish for about 3 or 4 seconds, then I withdrew the brush to just clear of the surface and allowed it to drip gently into the can until no more drips fell from the brush.

The instrument was set horizontally on the covered bench with the back facing upwards. I quickly and smoothly applied the finish, starting at the top about two inches in from the side of the upper bout. The first stroke was extended from about two inches from the top to two inches from the bottom. This column of finish was then brushed out to cover the two inch uncovered strips. This procedure was adopted to prevent the finish from flowing over the binding running down the sides.

The second stroke was started two inches from the top and continued down to two inches from the bottom. This stroke was made to slightly overlap the first stroke. The uncovered areas at the top and the bottom of this second stroke were then brushed in. Subsequent strokes followed this procedure until the top was covered completely. Please note that the brush should always move from a wetted surface to a dry area-the same as if you were painting an interior house wall.

As the finish was applied, I kept recharging the brush by dipping the end of the bristles into the can for a couple of seconds and allowing the excess to drip off the brush back into the can. Using this method, I avoided creating bubbles by working the brush too much.

It is very important to use a strong back light to see where the finish is going, and one has to be careful not to miss a spot. It is also important to charge the brush with the correct amount of finish. This comes with experience.

I did not brush back and forth but laid down the finish as smoothly as possible, trying to have the finish flow onto the surface. If I missed a spot, I did not try to rebrush it.

 When I finished coating the back of the guitar, I moved to the sides. I held the guitar by its neck and coated the sides by working down from the heel of the neck to the butt of the guitar-first one side and then the other. The guitar was held vertically and the strokes were made from the center of the sides to the back and from the center of the sides to the front at about a 45 degree angle.

 It was important to work quickly and smoothly. I did not to try to touch up any area because this would have resulted in a streaked finish. The surface was covered once, then was allowed to dry.

I applied three coats of finish each day for two successive days and allowed the finish to dry for a minimum of two hours between coats. An infra red lamp can speed up the drying process if you have one. If there were any gross drips, I sanded them off with 400 wet/dry sandpaper before I applied the next coat.

After the final coat was applied, I allowed the finish to dry for a couple of hours, then I sanded. The instrument was then allowed to harden for four weeks before final sanding, buffing and polishing.

The instrument was wet sanded with successive grades of sandpaper starting with 400 grit Mirka wet/dry paper using a flexible rubber block as a backing. When I was satisfied with the overall appearance of this layer, I moved through finer grits of wet/dry Mirka. I used 600 grit, 1000 grit, then 2000 grit successively. As the water was applied to the surface during the sanding operation, the beauty of the wood grain started to jump out at me and I began to have a feel for what could be expected in the completed instrument.

Finally, although not strictly necessary, I used a dry 4000 micro abrasive because I thought that it would save me some time during the buffing operation. This followed John Greven’s recommendation.

I had to build my own buffing machine. I bought a buffing arbor, two 12" diameter muslin buffing wheels, two 12" diameter cotton (Domet) buffing wheels and two large steel washers to support the center of the Domet wheels from LMI. I also bought a 1 inch diameter pulley to mount on the motor shaft. In addition, I bought a stick each of Menzerna brown course (600 grit) and white fine abrasive. From Brazil Electric, I bought a 1 1/3 HP, 1725 RPM electric motor. All of this stuff was assembled on a plywood base, which was clamped to a heavy wooden stand that I move outdoors for the buffing operation. The center of the muslin wheels were flooded with cyano acrylate, and after the glue had dried, power sanded to ¾” diameter to fit the arbor shaft. The glue causes some smoke when dropped onto the cotton. The pulley ratios gave me a buffing speed of about 1100 RPM, which is about the upper limit for buffing, but gave me no problems.

Buffing was a fairly easy operation and only took about 15 minutes. The Domet wheel was charged with the brown coarse compound and the Muslin wheel, with the white fine compound. Each wheel was dedicated to each compound. A strong light was used to keep a beady eye on things.

Important note: A buffing machine is an efficient transportation system, which will move your new guitar, in the blink of an eye, by air, from one end of your shop to the other, so be sure to have a firm grip on the your instrument before you start to buff.

The buffing wheel will rotate toward you, so hold the instrument slightly below the level of the axle. I gradually brought the instrument surface to bear on the wheel charged with the coarse compound. I worked down the spine of the instrument out towards the edges of the bout. The back was buffed this way. I repeated at 90 degrees, then again at 45 degrees. Do not present the edge of the instrument directly into the buffer because you will burn the finish off the edge Do not use a lot of pressure on the wheel, a somewhat light touch is all that is required. The back of the neck and the sides were buffed next.

The same procedure was followed using the fine buffing wheel.

You may hand polish or you can power polish using Abralon sanding disks. These are available from LMI.

I rate this first attempt as highly successful and I am pleased with this finish, which came out very clear and transparent. Application was far easier than I had thought and, with more practice, I expect to become very proficient at using the brush to apply this finish. The two most difficult parts of the process were judging the correct amount of finish to charge the brush with, and the method of brushing out the finish onto the instrument. This was, however, a matter of experience and I quickly found way of handling this finish.

I suggest that before trying to finish an actual instrument you practice on some scrap plywood to get your feet wet and refine your technique.

 Give it a whirl! You will be delighted.

Waterborne Finishing with Grafted Coatings "KTM-9"
by Mike Doolin

Please note: As of January 2006, I have switched to catalyzed polyester and am no longer using waterborne finishes on my guitars. I have not used a waterborne product since then, so I'm sure my knowledge of them is hopelessly out of date. I will not be updating these instructions any further, nor will I answer email questions about them. Such as they are, these instructions contain everything I know (or rather, knew) about waterborne finishing.

Why did I switch? Hardness, and speed. Polyester is pretty much the standard finish on factory guitars, and it provides the durability to which most steel-string players are accustomed. And since it's basically 100% solids and cures by catalyzation instead of evaporation of solvents, it doesn't shrink back over time. I can rub out a finish after 5 days cure and know that a year later it will look just as good.

But those qualities come at a price, and that price is toxicity and flammability. I spent about $2000 on a commercial spray booth, built an explosion proof room around it, and vented the booth out through an unused utility chimney. Plus, I bought a supplied-air full-face respirator. With solvents like acetone and MEK and with MEKP as the catalyst, this stuff is not to be trifled with. Besides, it stinks to high heaven until it's cured.

And so, I still recommend waterborne finishes to small-production and hobbyist luthiers, and anyone unwilling or unable to make this kind of investment in finishing equipment. But I would recommend asking around about waterborne products which have come out in recent years, it's likely that improvements have been made, and I haven't kept up on the technology.

Finishing is the Achilles Heel of lutherie. Lacquer, the standard by which all others are judged, is clear and tough but toxic and flammable. Waterborne finishes try to solve the problems of toxicity and flammability, but never seem to be really clear and really tough. We've all tried the waterborne products offered for musical instruments and found them wanting.

When I decided to build guitars full-time I knew I'd have to find a waterborne finish which really did the job. I wanted to work from my home, which meant working in the basement next to my gas water heater and furnace, which meant that highly flammable finishes were not an option. So I bought a little of every waterborne finish I could find, from hardware-store hobbyist products to commercial acid-catalyzed cabinetmaker's finishes. Overall, the quality was shockingly low, and the best of the bunch were mostly those already offered to luthiers.

But in recent years, waterborne technology has advanced tremendously, and I've tested and sometimes used new products as they've come out. The latest of these is Grafted Coatings "KTM-9". It dries hard and clear, can accept water- or alcohol-based dyes for tinting and colors, and rubs out to a high gloss. It builds, self-levels and burns into itself well. It has low shrinkage after curing and leveling, and is resistant to sweat. The resulting finish is the closest I've seen to traditional lacquers and varnishes.

I spray in my small shop wearing a mask, near a 12" fan blowing out a window. I wear a glove on my left hand since I hold the guitar with it while spraying with my right, to avoid spraying my skin. I don't have a spray booth, I'm spraying in a corner of my main shop space, so I run my air filters for a few minutes before and all day during spraying, to keep the wood dust out of the air. The space has no humidity control and not much temperature control either, so actual spraying conditions vary from about 60 to 90 degrees Fahrenheit, and 35% to 85% relative humidity, depending on the time of year. However, I have a humidity controlled storage room where I put all works in progress; the humidity is always 45% in there, while the temperature varies from about 75 to 85 degrees. I hang pieces in there immediately after spraying each coat, so they dry under controlled humidity conditions. Then after all spraying is complete, I leave the sprayed pieces in that room while they cure.

I've developed a method for applying waterbornes which has been giving me very good results. I commonly hear luthiers tell me that they assumed my finishes were lacquer! This stuff works, if you play by its rules. So here's my method as of this writing...

  1. Fill pores and seal. I use System Three SB-112 Epoxy as a sealer-filler, thickened with their "Silica Thickener". It "wets" the wood for good color and depth, and provides some dent resistance that the waterborne top coats lack. I apply the epoxy to all the surfaces, even the top! If applied thinly, I don't believe there is any dampening of sound quality. Since both the epoxy and the waterborne polyurethane are alcohol-soluble, the waterborne adheres to the epoxy extremely well. I've actually tried several epoxies, and all have worked well.

    One exception is cedar. For some reason, epoxy can make some cedar tops look blotchy. One solution is to seal cedar tops with a sprayed coat of thin shellac instead, and epoxy everything else. Don't put both shellac and epoxy anywhere though, they don't stick to one another, but waterborne sticks to either one very well. I seal my spalted maple rosettes with thinned white glue before the epoxy for the same reason - epoxy darkens the spalted areas too much.

    The mix ratio is critical for epoxy; if you get it wrong, it will never cure and will leave a sticky mess. I measure it by weight on a triple beam balance gram scale since I'm using such a small amount. The ratio by weight is 2.23 parts resin to 1 part hardener. I thoroughly mix the epoxy first, and then mix in an equal amount by volume of Silica Thickener. I only mix up about a tablespoon of the mixture at a time. The mixture is about the consistency of cold cream, and almost that white in color, but it goes invisible when scraped into the pores.

    However, epoxy is not intended as a finish product, so it doesn't flow out well at all and tends to fish-eye. The solution to this is to scrape it on with a plastic card, a business card or a single-edge razor blade, leaving as little as possible on the surface. I spread the epoxy across the wood surface and immediately scrape off as much as I can. This is a critical point: scrape off as much epoxy as possible! Don't leave any on the surface, just scrape it away and it will remain behind in the pores. That way, you won't have to sand it level later. On the other hand, make sure to get some epoxy on every square inch of the wood surface, to ensure consistent color and adhesion under finish.

    The curves of the neck are a difficulty. I scrape it on with a flexible plastic card that will wrap around the curves, and scrape off as much as I can that way. Then I wipe off the little ridges with an alcohol-wetted rag.

    The epoxy starts to set in a few minutes, so I typically mix one batch for the sides, another for the back and top and another for the neck. So, a few tablespoons of epoxy is plenty to seal and fill an entire guitar. If you do a careful job of scraping, making sure to cover every square millimeter but not leave any on the surface, you will completely fill the pores in one application. In practice, I almost never achieve a one-coat fill, so I apply another coat after at least 6 hours curing in my storage room. Temperature affects the cure too, so it's best to hang the guitar in a fairly warm room while the epoxy is curing, 70 degrees or higher.

    Important: do not use shellac over or under epoxy, the two will not stick to one another! Either one works as a sealer and improves adhesion to the waterborne, but it's one or the other, not both. I prefer epoxy because it "wets" the wood better and acts as a filler as well.

    After curing over night I may sand the surface very lightly with 400 grit gold sandpaper to remove any little nubs or dust specs. This is a very cursory sanding - you don't want to sand through to the wood, you just want to level the surface and take off any nubs. It's even better if you don't have to sand at all, which is possible if you do a good job of scraping the epoxy off when it's still wet. If you sand through to the wood, it won't have the same color or depth as the rest of the surface. If you do sand through to the wood, you can thin a little epoxy with an equal part of denatured alcohol and wipe this thinned epoxy onto the surface with a soft cloth pad, wiping with the grain and trying not to go back over areas you've already wiped. The alcohol evaporates away quickly, leaving behind a very thin coat of epoxy. The purpose of this coat is to get some epoxy onto any bare wood that you've exposed, to ensure consistent color and adhesion. Let this last epoxy coat cure for at least 6 hours before applying the waterborne.

  2. Spray the first day's worth of waterborne coats. I spray 5 coats in a day, at hour intervals. I use a cheap touch-up gun from Harbor Freight) with my regulator set at 40 lbs.

    It's important to filter any waterborne finish, and most commercially made filters don't work well with waterbornes. I use a gold metal coffee filter in a wide mouthed funnel. I filter the finish directly into the spray gun's cup, immediately before spraying. Waterborne finishes form little clots constantly, so I filter just enough finish into the cup to spray one coat, and clean the gun between coats. I flush the gun with water and leave water in the gun between coats, to prevent the finish from building up inside the gun. When I'm done spraying for the day, I flush the gun a second time with denatured alcohol, and leave the alcohol in the gun until the next time I spray.

    The correct coat should look a little orange-peelish at first - thicker than this and you risk sags and milkyness. If you spray too thin the surface will look grainy, which also hurts clarity and adhesion. Definitely don't spray it on thick enough to see any milkyness in the wet coat - this will almost certainly sag, and won't dry clear and hard.

    After spraying all surfaces of the instrument, look it over for sags in the making. If you see any, the best thing you can do is to wipe them away with a finger while still wet. You can sometimes tilt the surface this way or that to prevent a sag from forming, but it's much better if you don't spray thick enough to cause them in the first place. Once dry, a sag will be rubbery for days and possibly never completely cure. You can sometimes slice them off with a brand-new razor blade, but frequently the sag will tear the finish all the way down to the wood. This can be drop filled with more finish, but again it's much better to spray thinly enough to avoid the sag in the first place.

    Assuming no problems with heavy-handed spraying, you can spray 5 coats at hour intervals in one day. I hang my guitars to dry in my wood room between coats, where I maintain 45% humidity year-round. I have a fan running in the room to keep the air circulating. This is very important to getting the finish to dry clear! Watch the clarity of the film, and if you start to see milkiness after an hour's drying, you've probably sprayed to heavily or your drying environment is too warm or humid.

    Cleaned out the gun, first with water and then with alcohol. Leave the last of the alcohol in the gun, this will keep residue from building up.

  3. Drop fill and level the next day. Hopefully, if you've done your epoxy coats well, there won't be any gaps or pores to fill. Waterbornes seem to abhor bridging small gaps like big pores or purfling line gaps. While you can do drop fills with finish, it usually just soaks down into the gap, seemingly indefinitely. CA glue works much better, and is completely invisible between layers of finish. Medium-viscosity and "brushable" super glues are especially handy for this. You can squeegee these into the gaps with a razor blade, leaving very little above the plane of the finish. Note that while you can scrape super glue level once it's dry, waterborne fishes do not scrape well. Better to squeegee the wet super glue level and sand once dry. However, super glue accelerator can turn bright yellow on some woods under the finish, so be sparing with the accelerator.

    Don't apply super glue over freshly sanded finish. The super glue won't melt the sanding scratches and you'll see them under the finish. Always drop fill before leveling, and sand away all the super glue except the part down in the gap you're filling.

    You can also mix some Silica Thickener into KTM-9 and drop fill with that. I've gone so far as to casually wipe some thickened KTM-9 into the pores with my finger, and them sprayed the day's coats over that. You'd never see it in the final finish. A business card works well for scraping on thickened finish to fill large areas. Just like with thickened epoxy, the goal is to leave as little as possible on the surface.

    Once you've filled any little gaps you can sand level. I sand back with dry 600 grit Mirka Q-silver paper. I do most of my sanding with random orbit sanders. I have three: a 5" DeWalt electric, a 3-1/2" Stuhr air, and the 2" Harbor Freight air. I use the 5" for tops and backs, the 3-1/2" for sides, and the 2" for waists and cutaways. For final leveling I use the 3-1/2" for tops and backs as well for extra control. On neck shafts and heels I use a 1/4-sheet Porter Cable palm sander. About the only hand sanding I do is on the edges and the heel cap.

    Before spraying more coats, always wet the leveled surface down with denatured or grain alcohol. This melts out the sanding scratches and makes the surface a little tacky, to ensure the next coat "burns in" to the previous day's coat. If you don't do this, you risk incomplete adhesion, witness lines, and visible sanding scratches under the finish. I use a second spray gun for this melt-in step. I fill it with alcohol and spray the guitar as if it were finish. Any spot that doesn't get alcohol may not adhere!

    You can use the same spray gun you use for finish for this step. If you've sprayed the day before you should have cleaned out the gun and left alcohol in it to keep residue from building up. Spray that alcohol on the guitar, then by the time you've filled it with finish the alcohol will have dried and you can spray away.

    Then spray the final 5 coats of finish at hour intervals. I strongly recommend applying each day's coats within 24 hours of the previous day's coats. I've had adhesion problems when I let a day pass without spraying, even though I wetted the surface with alcohol after sanding.

    The thickness of a sprayed coat can vary tremendously depending on spray technique. Each luthier should experiment with how much finish to apply, to find the minimum amount that can be leveled and buffed without going through. Less is more, to a point, but I encourage everyone to eschew the "my finish is thinner than your finish" game. Put on just enough to reliably level and buff out, and it will be plenty thin.

  4. Let cure. KTM-9 cures quite quickly, but I give it 2 weeks when possible, 5 days at minimum. You can rub out sooner, but you may see the pores reappear later. I did a test panel of black walnut which I finished as I've described, and then rubbed sections out at 2, 3, 5, 9, and 14 days. I found that at 2 to 3 days there was too much shrinkage; at 5 days, shrinkage is acceptable; at 9 days there's little difference from 5; and at two weeks, there's virtually no shrinkage. Heat and air circulation are the keys to quick curing, so I crank up the fan and the thermostat in the room.

  5. Level and buff. After curing, the previously leveled surface will have shrunk considerably. I level the cured surface with dry 1000 grit Mirka Q-Silver paper, followed by 2000 grit Mirka Abralon. Note that I don't do any wet sanding! Both of these abrasives are non-loading. I use a new piece of Q silver abrasive for each plate: one for the top, one for the back, one for the sides, one for the neck. The Abralon is an abrasive cloth backed with foam, and works best with very light pressure, allowing the random orbit sander to freely spin. Depending on how patient you are, you could sand again with 4000 grit to leave less to do with the buffing wheel.

    I buff with 12" wheels spinning at about 850 RPM. I use Menzerna coarse compound to take out the sanding grit scratches, medium and fine compounds to take out the buffing scratches, and 3M Finesse-It II on the random orbit machines for final polish. I really load the wheels up with compound, adding more after each full-surface pass unless a lot of compound is being left behind on the surface. The coarse compound does all the work; the medium then removes the coarse compound scratches and the fine removes the medium compound scratches. The better a job you do with the sanding the easier a time you'll have at the buffing wheel. If you try to do too much with the wheel you'll heat the finish up, causing shrinkage which brings the pores back out. Never, ever, buff into an edge! You can burn through an edge in an instant. You can repair small edge burn-throughs with super glue, alcohol or shellac, but best to avoid them in the first place.

    I press moderately lightly into the wheel and keep moving to avoid building up heat. I buff with the wheel going fairly fast, 2:1 ratio on the pulleys with a 1700 RPM motor. Each pass across the surface of a back or top takes about one second. I always cross in the same direction, rather than back and forth, again to avoid building up heat. Starting with the coarse compound, I go over a surface from end to end with these overlapping passes, recharge the wheel, then turn the guitar 90 degrees and do the same surface again, recharge, then turn 45 degrees and buff again, recharge, turn 90 degrees once again, recharge, and then buff off of (not into!) the perimeter all around. Then I finish the coarse compound step with several light passes with the grain, without recharging.

    I hold the surface up to reflect the light from a bare light bulb and try to see what surface scratches are present. The sanders leave small circular scratches while the wheel leaves long straight scratches. I've buffed in 4 directions with the coarse compound, but now I want to take out everything but the scratches going with the grain. By orienting the surface to the light I can easily see all the various scratch patterns, and I continue buffing with the grain until no cross-grain or circular scratches are left.

    Then I buff across the grain once or twice with Menzerna medium compound, until all the coarse scratches are out, and once or twice with the grain with the Menzerna fine compound, until the medium scratches are out. Each time, I use reflected light to see the previous step's scratches. I go around the corners with a soft cloth at the end of each compound, using the residue there to buff the corners. The whole buffing takes about an hour for one guitar. I dedicate one wheel to each compound, so since I have a double buffing arbor I have to change the wheel from medium to fine.

    The final polish with Finesse-It II removes the straight scratches left by the fine compound. I put a tiny drop of Finesse-It II on the wood and work an area about 6" square using foam pads on my 3-1/2" and 2" random orbit sanders. The compound must be worked until it dries and no residue is left on the surface, which is why I only put a tiny bit on. I work a small area at a time until I've polished the entire surface, removing all the straight scratches from the fine compound. Finesse-It II is available from suppliers of automotive and marine finish products.

    As I said above, if waterborne finish is allowed to dry for more than about 24 hours, subsequent coats may not "burn in" and adhere well. This can result in "witness lines" if you sand through the boundary, or silvery diffraction effects which ruin the clarity of the finish. Happily, alcohol will partly dissolve the surface if sprayed lightly the next day, allowing new coats to "burn in". The end result is a finish in which all coats have become one. However, once the finish has cured, it will not redissolve in alcohol or any other solvent, so spot touch-ups are difficult. The best way to repair the finish is to sand the entire surface and overspray it.

KTM-9 is water clear with no tint whatsoever. Though not necessary, a little tint can give a nice warm lacquerish tint to the finish if you want it. I like the Color Tone tints. These tints don't seem to affect the curing properties at all - I've done very intense transparent and opaque color finishes on solid body instruments with no difficulties. For a basic amber tint, I put 2 drops of Vintage Amber and 1 drop of Red Mahogany in a cup of finish, and spraying 2-3 coats of this adds up to a nice even tint. It's best to keep the color light enough that it takes several sprayed coats to achieve the final color - that will help you keep the color even. But it's best to only tint the first few of a day's coats, and spray a couple of clear coats over that. Then when you level you'll be sanding away just the top clear coats and leaving the tinted coats undisturbed. After spraying the second day's coats you'll have a full day's coats of untinted finish. This will prevent the color from bleeding onto the player's hands and clothes!

You can also spray Color Tone tints directly on the epoxy or the first few coats of finish. I add Color Tone to straight alcohol and spray that onto the finish. The tint actually soaks into the finish, and then I seal it in with more clear coats. This is a good way to create a sunburst. I spray my sunbursts during the first day of spraying KTM-9. After getting the epoxy perfectly level, I spray two coats of KTM-9 to provide a base for the sunburst. After that has dried for an hour, I spray many, many coats of the tinted alcohol, gradually building the color and creating a very even, subtle fade from the center to the edges without building any finish. Then I seal the sunburst in with straight KTM-9, spraying enough coats that I know I won't sand through to the sunburst when I level the next day.

Mike Doolin, Luthier