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Chemcraft Pore Filler Instructions


Chemcraft paste wood filler is a high solids alkyd resin filler that offers ease of use, high filling percentage, very little shrinkage and the ability to recoat in 12-24 hours. Working time and drying time vary based on temperature and humidity. It is a solvent based filler so you want to work in a well ventilated area, wear safety glasses or goggles, nitrile gloves and an organic vapor respirator is also a good idea.

Chemcraft filler can be colored very easily with universal tinting colors (UTC's). 10% by volume is a good tint ratio . At this ratio there is no worry about the filler changing color when dry or turning a grayish hue. It is recommended that you seal the instrument before pore filling. By keeping the sealer coat light, more filler will stay in the pores while the excess filler is being removed. These instructions are for mixing your own colors but if you buy pre-colored filler, start at the "Applying the Filler" portion of these instructions.

To do an entire acoustic guitar 300ml of product should be more than enough. Use 200ml of filler if you are creating a color for the back and sides and 100ml of filler to create a color for the neck. If you're doing an electric guitar body and neck, usually 200ml of filler is more than enough. 1 ounce is approximately equivalent to 30ml.

Here are the supplies you will need:

  1. Chemcraft filler
    a. Pre-colored or
    b.Neutral filler, FPFCN and Universal Tint Colors (UTC)  FTVDB, FTBU, FTBS, FTLB.
  2. Glass jars with air tight lids a. Pint sized canning jars work great
  3. Cheap 1" or 2" bristle brush
  4. Flexible squeegee
    a. Bondo applicators work well (FS)
    b. Scuff the edge lightly when new to prevent the wood from being scratched
  5. Measuring/mixing cups (FMC)
  6. Safety glasses or goggles (SG)
  7. Nitrile gloves (SNG)
  8. Stir sticks
  9. Respirator w/organic vapor cartridges
  10. Sandpaper (FFC)
  11. Sanding block
    a. Felt block (FPFELT)
    b. Wood with felt
  12. Brass bristle brush (optional)
  13. Toluene (optional)
  14. Burlap (optional)


Coloring the Filler

First stir the filler thoroughly then pour the amount you need into the jar and put the lid on until you're ready to add the colorant. As long as the filler is kept in an air tight container it will be usable for many years. Measuring everything milliliters makes it very easy to mix the amount needed for your instrument.

Here are some general mixtures that work for a wide variety of different tone woods essentially creating 3 different shades of "brown" (red, medium and dark). A black UTC is also very helpful when a very dark brown is needed. Add black in very small amounts until the desired color is achieved.

  • "Red Brown" is: 3 parts Burnt Sienna (BS) to 1 part Burn Umber (BU) — This is a good color for Padauk, Bubinga and similarly colored woods.
  • "Medium Brown" is: 2 parts Burnt Umber (BU) to 1 part Burnt Sienna (BS) — Use this on Mahogany, Koa or woods with a similar overall hue.
  • "Dark Brown" is: 2 parts Van Dyke Brown (VDB) to 1 part Burnt Umber (BU) Choose this for Rosewood, Cocobolo, Wenge and woods with a very dark color. 

Occasionally when a really dark brown (almost black) filler is desirable, mix equal parts Van Dyke Brown (VDB): Black (BLK):Burnt Umber (BU). Often I'll mix something like this if I'm applying color over the wood like toning a classic red color over mahogany.

Feel free to experiment as well to get the particular color you have in mind. Shake the bottles of UTC for 30 seconds or so just to be sure the pigment is dispersed evenly. Mix the UTC's together in a mixing cup until you achieve the color you're looking for and then add to the filler stirring thoroughly. Remember the color mixture should be added 10% by volume to your filler (200ml filler : 20ml color). By using a glass jar you can see when the product is completely stirred as there should be no lighter color of the natural filler visible anywhere inside the glass. Stir the product before each use and occasionally while you're working.
Example: To make "dark brown" for the back and sides of an acoustic guitar you'd want to pour out 200ml of filler (after stirring) into a canning jar and screw the lid tight. Now take the Van Dyke Brown and Burnt Umber UTC's shaking each bottle for 30 seconds or so. You want to make 20ml of color using a 2:1 mixture of VDB:BU which will then be added to the filler. Measure out 14ml of Van Dyke Brown to which you will add 7ml of Burnt Umber then stir the mixture thoroughly. While this 21ml is more than 10% by volume, some of your color will stay in the mixing cup after stirring and overall it's quite close. Once the color has been stirred well you can add it to your filler and again, stir, stir, stir until there is no hint of the grayish filler anywhere in the glass jar.

While it is not necessary to thin this paste wood filler you can work a larger area at a time if you do so. In testing I found thinning with toluene added 5-10% by volume worked well. If you don't want the hassle of thinning the product simply work in sections approximately 8" x 10" at a time. REMEMBER…temperature and humidity can change how finishing products perform! You can begin by using the product as is, then add 5% toluene if you feel it's too thick eventually adding another 5% by volume to thin it slightly more if need be. I can't imagine adding more than 10% toluene to the mixture would be necessary or even desirable.


Applying the Filler

Tape the instrument off as necessary. If the whole instrument has been sealed it isn't necessary to cover areas that don't need filler although it can't hurt. Any gaps at this point should be touched up before filling as well. If two different "browns" are being used tape off the heel while doing the back and sides and vice versa while applying filler to the neck. Other good ideas are to pack paper towels into holes for ferrules, tape off control cavities on electric guitars and cover the truss rod area to avoid lots of excess filler in areas that simply don't need it. If you have to sand areas that get touched up, re-seal that area before applying your filler to avoid a blotchy spot where filler colors bare wood.

Once the filler is mixed, your work area is clear and you are ready to begin, give the filler another stir before use. Work in sections until you get a feel for how the filler behaves. Its application is very similar to applying oil based wood filler.

Apply the filler with a cheap bristle brush across the grain enough so you no longer see the wood. When I use the slightly thinned filler I'll do the entire process to the back of a guitar, then repeat the process for each side sometimes even doing both sides at once. This all depends on temperature and humidity. Once the area is covered with filler, blot or poke the filler with the brush 90 degrees to the wood working the filler down into the pores also working air out of the pores to avoid having pitting or sinking of the filler. Do that to the entire area that has filler. Now wait until about 70% of the shine is gone as the filler hazes over. This can be anywhere from 5 – 20 minutes depending on the temperature and humidity at the time of application. The filler will "flash off" faster in hot, humid areas and slower in cold, dry environments. You don't want all the shine to be gone because that will generally cause some filler to pull out of the pores as you squeegee off the excess. It just takes a little practice to get the feel for when to remove the excess.


Removing the Filler

Once the filler has "hazed over" to where about 70% of the shine is gone, remove the excess with the squeegee at a 45 degree angle to the grain. This not only removes the excess put packs the filler down into the pores. Occasionally scrape the filler off the squeegee into another jar or onto a scrap of cardboard as you work. From time to time it is also helpful to wipe the squeegee clean with a paper towel or rag. Once the entire back is done then you can apply the filler to the sides repeating the technique. Having a flexible squeegee helps work the excess off at a 45 degree angle to the grain even on curved surfaces. There will be a very thin layer of filler left behind that could be removed with burlap or sanded off after 12-24 hours of dry time. Carefully scrape areas that have a lot of filler left on the surface the squeegee didn't remove. Don't scratch the wood but simply pull that clump or spot of filler off to expose what's in the pores so it can dry. Repeat this application/removal process for all areas of the instrument that need filler. If you want to use burlap to remove even more of the excess after you squeegee, wait another 15 – 30 minutes then rub across the grain. If the filler comes off easily it is too soon to burlap. It should take some elbow grease to get the haze of filler off the surface almost like scraping gum off the sole of your shoe. The drawback to using burlap is that it always pulls some filler out of the pores. If you see an area that doesn't look like it got enough filler you can re-apply right away using the same technique. The excess that has been removed can be left to dry and thrown away. Unused filler can be stored in an air tight jar and be reused until all of it is gone. With paper towels or rags it is always a good idea to hang them up to dry or dispose of them in an OSHA approved container. Never leave rags piled up on a bench or throw them away in a regular garbage can when still wet as they can spontaneously combust.

If you want to re-use the brush or clean the brush when switching from one color to another, simply have a small amount of toluene in a container specifically for this purpose. Before applying filler again blot out the excess toluene with a rag. Otherwise, once all the application is done just let filler dry in the bristles and throw the brush away. Any cheap .99 cent bristle brush will do the job.


Sanding the Filler

If you want to get the highest percentage fill possible avoid using the burlap and prepare yourself for some sanding. You can try sanding 12 hours after applying the filler using gold aluminum oxide paper like 3M 216U Fre-Cut Gold sandpaper (LMI Part# FFC*) and a sanding block with some sort of soft backing for the paper (felt blocks work well). Either use the same grit you used to prep sand the raw wood or go one grit finer sanding with the grain. For example, if you prep sanded the raw wood out to 180, use that again or go one step finer and use 220 grit. If the paper clogs right away it is too soon to sand. The filler dries with exposure to oxygen so wait another 12 hours or create air flow with a fan to help speed the drying time. Occasionally Cocobolo needs extra dry time as the oils in the wood can inhibit dry time. Once the filler can be sanded remove all the excess filler paying close attention to avoid leaving spots of filler or hazy areas. It is okay to sand through your sealer because once sanded, the instrument should be sealed again before top coating. Avoid excess sanding that can open up new pores in the wood.

The sandpaper will almost always clog some when sanding off grain filler so a brass bristle brush is handy for removing "corning" from sandpaper as you work. Those clumps of filler left on the sandpaper can scratch the wood which will show up when you spray another coat of sealer. Be patient, clean the paper often as you work and switch to new sandpaper regularly.

Once complete, vacuum or blow the dust off and check for clumps of filler in tuning machine holes or potentiometer holes and pick them out before spraying more finish. It's a good idea to re-seal the instrument with the same sealer you initially used. After you seal again check for any filler on the surface you missed and sand/seal those spots again before topcoating.


Other Considerations

Solvents in your topcoat can swell the filler as well as sink the filler if there is air trapped in the pores. If the filler swells simply let that coat of finish dry thoroughly and continue applying the finish. Don't be alarmed if you still see the pores or texture to the wood! Keep the first few coats of finish after filling very thin to cut down on these situations and realize it's normal to see some texture of the wood. Once you have all your coats applied and let the finish cure you will be able to sand the finish nice and level before buffing.

While this formulation of filler differs than the tan/brown Chemcraft filler I've used for years, this filler performs extremely well. It is gray as opposed to tan/brown and working consistency is slightly more viscous, but once colored & thinned (10%) it is nearly identical. Large pored woods can be filled with just one application when not using burlap and with its ease of application and fast dry time; even double filling can be done in 2-3 days as opposed to the long dry times of an oil-based filler. It also colors much easier than water-based fillers that tend to turn colors pastel or very gray when dry. The only concern may be Chemcraft's solvent content but as long as your work area is well ventilated and safety glasses, gloves and respirator are used, the trade off is well worth the quality of the finished product. Remember to follow local ordinances regarding disposing of finishing materials or solvents, lay rags off your bench or hang them up to dry before throwing them away. You can even dip them in water before hanging them up to dry. Filling open pored woods is labor intensive just about any way you approach it. For me though, paste wood fillers are still the best approach. This product is one of the best I've ever used and I'm always looking for a more efficient method or product. By purchasing the neutral colored filler and UTC's you can save costs and avoid having a lot of excess product sitting around until your next build is ready for finish. It also allows you to color the filler any way you want! Just remember to use approximately 10% color by volume in relation to the amount of filler you are preparing.