I've been in the process of finishing guitars and have been thinking a lot about finishes lately, so I thought it would be appropriate to write a little on the subject. I'll try not to get too geeky or technical.
Finishing is the most tedious part of the guitar building process and I'd consider it the most frustrating. It seems like, at any moment, things could go terribly wrong. That's one way to look at it. You can also look at it as the most rewarding as things seem to come to life. The finish brings out the grain and figure of the wood and adds depth and character.
There are all kinds of finishes on the market today and most guitar makers use types that they spray on. Some use water based finishes, some used catalyzed lacquers, some of the folks who build traditional style instruments use nitro cellulose lacquer. Nitro Cellulose lacquer became popular in the 30's and remained the main finish through the 1980's. It goes on very nicely and each coat melts into the next. It flows out nicely and gives good color to the work. The downsides are that it cracks over time and it is extremely toxic and flammable. I have used lacquer in the past but have stopped, mainly because it makes me physically ill and I don't want to have anything that is that combustible in the shop. I also don't want to blow the exhaust fumes all over my neighborhood.
I'm always trying to figure out the ways that the old guys did things, and finishing is no different. From working on a wide variety of pre 1935 guitars, it seems like most of the old builders used varnishes or shellac. These finishes were applied either with a brush, or by a process called French polishing (which is a lot like spit shining a shoe).
Shellac is a resin which is secreted by a bug called a lac beetle. Folks in India and Thailand collect that resin and process it and turn it into flakes. The flakes come in a variety of colors, ranging from a very blond amber to a brown orange. The flakes are dissolved in alcohol and that is what is used as a finish. Shellac is great for a variety of reasons. First off, it is completely non toxic (I usually add the flakes to 190 proof vodka, Everclear). Second, it provides wonderful color to your work. It can turn a fish belly white spruce top into a beautiful pumpkin color and make things look instantly old. Third, it dries very quickly and is easy to apply and work with. Finally, it is very easy to repair, as long as you know what you're doing.
The downside to shellac is that it doesn't hold up to water. My response to this downside used to be, "Don't get your guitar wet." I have since learned that some folk's sweat can cause the shellac to get a little rough, and occasionally you are caught in a rainstorm, in a foreign country, with a guitar, and despite the fact that it's in a case, it can still get wet and the finish can get damaged. It can also be difficult to find the materials needed to repair a shellac based finish in a foreign country. How do I know this? It's a long story and I'm not going to get into it now.
Shellac does eventually harden and become more resistant to alcohol and water, but that takes a very long time. Like 20 years.
Varnishes come in two different kinds, spirit based and oil based. Spirit refers to alcohol and technically speaking, shellac is a spirit based varnish. Usually though spirit varnishes are shellac based with other resins added to them to make them more flexible, and heat or water resistant. Spirit varnishes can be applied with a brush or by French polishing. They dry very quickly and you can apply several coats in a short period.
Oil based varnishes are not as quick to dry as spirit based varnishes. As a result they need to be applied with a brush (some folks apply them with a spray gun), and more time is needed between coats. The up side to this is that the varnish flows very nicely and it also provides more protection than a spirit based varnish.
For a long time I French polished my guitars, first with straight shellac, then spirit based varnishes. This was very time consuming but it was perfect for working in a small shop with no ventilation and the results were terrific. As I gained more experience, I started looking into different options that provided a little more protection. I briefly used lacquer, but as I said, it made me sick and I didn't like the thought of using something that toxic.
I've always been curious about varnishes but there is very little information out there on varnishes and the whole subject is very intimidating. I had heard about some guitar makers using "varnish", but upon further inquiries realized that they were actually using polyurethane. I have never been crazy about polyurethane, the depth and color of the finish doesn't appeal to me, neither does that fact that if you sand through a coat, you get "witness lines".
I started to talk to some of the mandolin builders who use varnishes. Both Don MacRostie and Lynn Dudenbostel were extremely helpful on this front. Both Don and Lynn recommended alkyd resin oil based varnishes. Don sprayed his, Lynn used a brush. I should add that I did my first instrument work when I was 19. I rebuilt an upright bass with an old violin maker. When it came time to finish the bass, he recommended that I use Pratt and Lambert #38,an alkyd resin varnish. I did a competent job on the bass and it has held up through all sorts of conditions, playing on the street, in touring bands and now it is exposed to all my kids can dish out. It's held up wonderfully. It's funny to thing that after all that experience and research I've come full circle.
I had read somewhere along the line that the folks at Gibson used to use oil based varnishes, applying several coats, and then French polishing shellac over the top. I started trying this technique out and it works beautifully. The shellac adds to the depth of the varnish, it goes on really quickly and builds up a beautiful shine. So currently that is what is working best for me.
So, for all of you out there who are guitar builders, either professional or hobbyists, I encourage you to give varnish a try. It's not nearly as difficult as it seems and you don't have to wear a space suit to brush it on. In most cases you can get some good stuff at the local hardware store, and you can't match the beauty of it. If you have any questions, send me an email and I'll try to answer them.