For my current electric guitar build, I've decided to try out Target's Emtech EM6000 water based acrylic lacquer as a clear top coat. In the past I've used Hydrocoat's Resithane Plus with great results, but I wanted to try EM6000 because of it's burn-in capability, which the Resithane Plus lacks. What this means, is I won't have to sand between coats. That feature alone will mean a lot of time saved.
Instead of spraying on the EM6000 with an HVLP sprayer as I normally would have done, I decided to brush it on since other luthiers have had success with this approach. Water based lacquer in my opinion is harder to spray than nitrocellulose lacquer because it take longer to tack up. For that reason, WB lacquer has a greater tendency to run or sag before it sets up. This can be a real problem with all of the curves and angles found on a guitar.
Brushing WB lacquer, however, is not full proof. Regardless of the type of brush you use or whether the lacquer is thinned with either water or retarder, you can expect a significant amount of air bubbles in each coat you lay down. Fortunately, I chanced upon a technique which eliminated the air bubbles completely. I'll explain in a moment.
The first step in applying the top coat is to make sure the surface is ready. That means it has to be smooth with all of the grain and pours filled in. I started the prep work by sanding the guitar with 220 grit paper to get the surfaces nice and smooth. Because the body of the guitar is made from Honduran Mahogany, which is an open pour wood, I had to fill the grain. To do this, I wiped on a liberal coat of boiled linseed oil and sprinkled on some 4f pumice. The pumice turns clear on contact with the BLO and as I worked it into the wood, it generated wood dust to form a paste, which fills the pours and grain. I used my bare fingers to rub the paste with a circular motion into the wood. After the entire surface had been rubbed with the BLO/pumice/wood dust mixture, I let it dry over night. The next day, I wiped off the excess grit with a tack rag. Then I let the body dry for 4 days. The neck on the other hand, is made out of Maple, which doesn't require grain filling.
After the 4 day drying period, I was ready to start brushing on the WB lacquer. The process is simple. I dipped my soft, 2" wide brush into the can and spread it over the body and neck as if I were painting it. Since WB lacquer is rather runny, I had to be conscious of runs, especially down the sides. This was no problem as I caught them when they formed and brushed them back into the rest of the wet coat.
At this point, I was dismayed by the formation of thousands of tiny air bubbles. In many spots, the lacquer actually turned foamy. To rectify the situation, I immediately grabbed a small 6" square piece of clean, 100% cotton cloth, moistened it with water and formed it into a ball. With the first coat still wet, I rubbed the cloth back an forth in a pendulum motion from one end of the wood to the other sort of like a French Polishing technique. This effectively removed the air bubbles and spread the coat evenly over the surface. The only downside to this technique is that it leaves subtle brush marks in the surface. However, since I knew I would have to level sand the surface before buffing, I wasn't concerned.
Each coat went on about 30 minutes apart and after 16 coats, I decided that was enough. The instruction from Target Coatings recommends letting the finish cure for 100 hours before polishing. However, with so many coats, I decided to let the body and neck cure for a week.
Level sanding consisted of dry sanding the body with 800 grit silicone carbide paper. Dry sanding at this early stage of leveling allows you to better see what's happening on the surface. The WB lacquer turned to white powder as I dry sanded it and when after wiping the surface with a tack rag, I could easily see the high and low spots. The goal was to sand the entire surface to a uniformly matte appearance. It happened very quickly and I only had to apply very light pressure.
Once I was satisfied the entire surface was level, I began wet sanding very lightly with 1,500 grit paper dipped in a mixture of water with a few drops of Murphy's oil soap for lubrication. Then I switched to 2,000 grit to finish. At this point, the surface had become quite reflective. However, it wasn't shiny enough. The buffer would take care of that.
I buffed with two 12" soft flannel wheels. On the first, I used Menzerna medium compound to remove any remaining fine scratches left by the level sanding. Very light pressure is the key as too much will overheat to lacquer and cause it to blister. On the other wheel, I used extra fine Menzerna compound, which gave the body the much desired wet look.
I'll shoot some photos when time permits and post them here for you to see the results. Stay tuned!