Please note: As of January 2006, I have switched to catalyzed polyester and am no longer using waterborne finishes on my guitars. I will not be updating these instructions any further, as they represent the best techniques I am aware of for finishing guitars with waterborne.
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, particularly KTM-9, to small-production and hobbyist luthiers, and anyone unwilling or unable to make this kind of investment in finishing equipment. KTM-9 is still a fine choice as a finish for new handmade instruments.
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 flamable 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 milimeter 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 surfacevery 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 suface. 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 consistend 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 ($19.95 at 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 gune 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.
Grafted Coatings, Inc.
Mirka Abrasives, Inc.
Stewart-MacDonald (for Menzerna compounds)
Back to my waterborne finishing page