John Whiteside replies: After filling the air with poisonous vapors and running the risk of explosions using sprayed nitrocellulose lacquer, I searched for a water-based alternative that would work well for the impeccable, high-gloss finish required on guitars. Fortunately there is such a product -— KTM-9.
This water-based lacquer is clear, non-flammable, not known to be toxic, and dries to the touch in less than an hour. It cross-links into itself when an additional layer is applied provided that the next coat is put on within 24 hours of the previous coat. That means no sanding between coats is required. If you wait more than 24 hours between coats, then a light sanding is required (as is the case with varnish) to get the new coat to adhere well.
As is the case with lacquer, if the plan is to buff the finish to a high gloss, it is wise to wait for a week to let the finish harden before the buffing. Sanding between coats, if required, can be done without the wait.
It is possible to apply with a brush. One expert recommends foam brushes. Another favors a high-quality bristle brush. I prefer to spray it and have had good results. My spraying setup involves a good-sized compressor with an oil filter installed in the spray line. The spray gun is a simple, inexpensive, gravity-fed cup sprayer. I have had the best results running the sprayer at 40 lbs of pressure.
The material is surprisingly resistant to runs and drips; you really have to load it on to get a run or sag. Some practice is required to hold the gun at just the right distance from the surface being finished. Too far and you get orange peel, too close and you might get runs. I set up a bright raking light that shines across the surface so I can see when just enough spray has been applied.
If I am doing a series of coats, say four in a day spaced two hours apart, I do not clean the gun after each coat, Simply putting the gun in a zip-loc bag works well. When spraying is done for the day, I clean the gun out with alcohol.
The finish is absolutely colorless so my technique has been to put down any stain or dye first. Also, I always seal the wood with shellac. KTM-9 adheres well to shellac, just make sure the top shellac coat is dewaxed.
The degree of perfection and glossiness in your final product is a question of how much you sand. I typically wet-sand at 320 (or 400), 600, 800, and 1000 grit and then switch to the buffing wheel. The wheel is a floppy muslin wheel, 12˝ in diameter, rotating at no more than 900 rpm, and impregnated with fine abrasive. This produces a show-room quality finish than has stood up very well to daily handling.
The main drawback is expense — KTM-9 costs about $30 a quart, though the price goes down if you buy it by the gallon. It is available from Luthiers Mercantile International (www.lmii.com). Look under “Finishing.”
Peter Breu replies: I have been spraying Target Coatings “Emtech EM6000” which is a water based acrylic lacquer for three years now and really like it. I was using traditional nitrocellulose lacquer before and was very tired of the powerful solvents. It is very easy to spray – straight from the can gives great results. It levels very well, dries quickly (I can recoat in 90 minutes) and the clean up is amazingly easy – just soap and water. You can’t leave it in the gun overnight, but the clean up is so easy that is really not a bother.
I have done a number of large and small projects with it and won’t go back to solvent based lacquers. It is available from Maine Coast Lumber (207-363-7426) www.mainecoastlumber.com for $40 a gallon.