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THE FINISH LINE by Rolfe Gerhardt

Excerpted from "the Finish Line" that ran in the Summer 2008 ASIA Guitarmaker magazine.

I found that KTM-SV applies easily, forms a hard but flexible finish, is wonderfully clear, buffs quite well, shrinks almost not at all after curing, and should not be overlooked by instrument builders. The main drawback is that, being a harder finish than I had been using, it takes longer to sand. Applying KTM-SV involves the usual stages of preparation. It is important to minimize grain raising by dampening the wood between sanding grits. I rough sand with 120 then go to 150, 180 and 220, the last three with garnet paper which produces softer sanding scratches. Sandpapers with stearates or silicon and tack cloths should be avoided.

The woods I use for mandolins don't require pore filling, so after the 220, I'm ready to stain.

KTM-SV, according to the manufacturer, adheres well to almost everything providing the base is fully cured. To avoid introducing air bubbles into the system, the KTM-SV should be stirred gently but thoroughly. If you are adding a dye tint, it is best to do that several hours before using and stirring quite thoroughly to allow full dispersion; I add two drops of amber mixed into 1 ounce of distilled water to each quart of KTM-SV for smoother flow-out. Always strain the finish when pouring it into the gun cup; a medium strainer works best for waterbornes. Again, according to the manufacturer, the temperature for spraying should be 65 degrees farenheit or higher for proper film formation. I never spray at less than 75 degrees. It is important to keep the finish from drying before it reaches the instrument surface, so the proper distance between the spray gun and the instrument is 4 to 6 inches.

Under normal conditions, the finish is dry to touch in 20 minutes, dried through in 40 minutes, and dried hard in 60 minutes, so, it can be recoated every hour. I plan my finishing schedule to get six coats on in one day, level sand the next day, then two more coats. One reason waterborne finishes have not jumped into popularity is the learning curve in spraying the finish. For mandolins, I like a spray pattern just over 2 inches high, for guitars, I would go 3-4 inches with at 50% overlap of passes with the spray gun. The pattern must be wet. How wet? This is the hard part of learning. Too wet and you get runs; too dry and you get a pebbly surface that doesn't flow. My only suggestion is to practice, practice, practice. It is helpful to have a brass brush on hand to scrub the build-up off the spray gun nozzle. A fiber brush won't touch it and a steel brush will eventually mess up the spray cap. Waterbornes have the bad habit of creating a build-up on the tip of the nozzle which narrows your spray pattern. It will do this several times while spraying a coat. I have a place to hang the gun so I can grab the brush and scrub the nozzle while still holding the instrument I am spraying. Also think through your spraying sequence so you minimize overspray. Oh, and do wear a spray mask because even waterbornes contain chemicals that should not be breathed. After each coat, clean the spray gun well.

A half and hour after spraying you can safely handle the finish, and that's the time to check for embedded nits to sand off, places to drop fill, or places that the spray doesn't reach well, like underneath the fingerboard extension on a mandolin. I use 800 grit sandpaper for any sanding or a scuff pad to knock off nits. Then I brush KTM-SV smoothly on these areas (it will adhere to nicely to the still-hardening finish already there). I generally lightly scuff the entire instrument after the first coat of KTM-SV, do the nit check after every coat, and do the brushing after the 3rd and 5th coats.

After overnight curing of the six coats of KTM-SV, I level-sand the finish using 800 grit. After level-sanding, I spray one coat and drop-fill or touch up any areas that need additional finish, and then after an hour spray the final coat. If I have sanded through any spots with the level-sanding, I touch up the color with a pen or colored pencil and brush some additional finish over the area before spraying the last two coats. If it has been more than twelve hours since the previous coat, I sand and then mist-spray the previous finish with alcohol, which briefly activates the dried finish. After the last coat, I usually let the finish cure two days before level-sanding again. The surface looks really flat and shiny but it needs another leveling. I use the same sandpaper as the first level-sand.

A day later, I sand with a Carborundum sponge pad in 1000-800 grit to take out the scratches form the level sand. A day later, I follow up with the Carborundum 1500-1200 sponge pad and sometimes Abralon 4000 a day after that. It is likely there will be some "witness lines" where you have sanded through one layer of finish into the next, but these will buff out. Buffing should not be done before 5 days, and is best after 7. Almost all shrinking and curing is done by then, but the curing goes on for a month according to the manufacturer. I power buff with coarse compound followed by medium compound. With guitars and some other instruments, further final buffing is required. With this finish, try not to heat the finish when buffing.

As with any finish, LMI recommends tests be done to practice pieces. Many factors influence a finish and as Rolfe said, "practice, practice, practice" makes perfect.