Poly is NOT the right finish for a guitar. Some of the big guitar companies are using UV cured polys right now (Taylor is one), but I think they look like crap, and you don't want to use a finish which requires you buy a ten thousand dollar UV light. The polys which are not UV cured do not dry hard enough, and they always look blue to my eye, which is ugly. The best finish for a guitar is nitrocellulose lacquer, which is what has been being used by most reputable makers for the last 70-80 years. It dries hard and clear, is relatively easy to spray, is very easy to touch up, and will polish to a mirror shine. It will also continue to off gas as it gets older, making it harder, which makes it sound better (this is one of the reason old guitars sound so much better than new ones, and also why you should think long and hard before refinishing and instrument). Some people do not like certain aspects of nitro. It will yellow in sunlight, and in time. I think this is a benefit, my self, as I think it gives the guitar class. Nitro will also, as it continues to off gas, continue to shrink, sinking into the grain lines of the wood. Again, I think this looks classy, and gives the instrument character, which is cool. The two problems with lacquer which are not questionable are it is highly toxic, and it requires a sizable investment in spray gun and ventilation equipment. You must have a good spray booth, with an explosion proof fan, and a fairly sizable air compressor with a nice spray gun to shot lacquer. You must also wear a respirator mask with filters rated for organic vapors, unless you don't mind cutting years off your life. It is not really a do-it-yourself finish.
Ideally, to spray lacquer, you start by sanding the bare wood to 180 grit, I like to use garnet paper. Then you wet the wood to raise the grain, and sand again with 180 grit, and then with again with 220. A professional finish starts with a good sanding job. Take your time, and get it right.
You want to start getting the finish on the guitar quickly now, as the wood will oxidize if you leave it too long, and then you will have to sand again to get good adhesion from the finish. If you want to stain the instrument, you do so now, following the instructions on the stain, and make sure it is compatible with your lacquer. The next step is to spray a sealer coat. The lacquer I use is made by a company called McFadden's, and they make a very nice vinyl sealer, which gives the finish some elasticity, which will reduce weather checking. I spray this on, and let it dry overnight. At this point, you want to look over the guitar very carefully to look for sanding scratches you missed before, as they will pop out after a sealer coat.
Next, I apply pour filler in porous woods such as mahogany, rosewood, ash, etc. Follow the directions on the pour filler, and make sure it is compatible with your lacquer. If it is not, you can spray a coat of freshly made shellac, in about a one pound cut. (Shellac is the wonder finish in this regard. It will stick to anything but Teflon, so if you need an intercoat sealer between two incompatible finishes, use thin, light shellac.) If you use shellac, this is your sealer coat, if not, apply another coat of vinyl sealer.
Once the sealer has dried for an hour, but less than 24 hours, you are ready for the first coat of lacquer. At this point, I will spray on six coats of lacquer, with at least one hour between each, though after the first one longer is not a problem. Let this dry for one week, and then sand with 220 or 320 grit fre-cut (stearated) paper, which lubricates itself as you sand. WEAR A DUST MASK WHEN YOU ARE SANDING, AS THE DUST IS VERY UNHEALTHEY. Find any dips or gaps in the finish, and fill them. Wait a day, and sand back the drop fills. You are then ready for the next coats of lacquer. I will spray four this time, and repeat the wait a week, sand and fill routine. At this point you should have a dead flat finish, but which is not yet ready for polishing. Next is two more coats of lacquer, followed by at least two weeks, and preferably a month of waiting.
You are now ready for wet sanding, which is one of the most annoying parts of guitar building. I have an air powered orbital sander, but if you don't you have to do it by hand. No electric sanders, as you are WET sanding. I start with a 600 grit wet or dry paper. When the instrument is completely sanded, wait 24 hours and do it again with 1000 grit. Now, you wait another couple days, and you can polish. I use a Baldor buffing wheel with Menerza stick polishes, and would never go back, but you can do it by hand, or with a large buffing pad in a buffer, using automotive polishing compounds. 3M makes the best, but they are expensive and hard to find. The Mirror Glaze stuff will take a little longer, but is cheaper, easier to find, and works fine. Work your way through the various levels of polishing compound. If you use a buffer, be VERY careful not to burn through, because though it can be fixed, it is time consuming and annoying. Once you have worked through all of the polishing compounds, you can reassemble your instrument, and you are done.
(If you ever wondered why it cost upwards of $500 to get a professional finish on a guitar, now you know. With the exception of hotrod builders, no one put as much care into finishing as guitar builders.
If you do not want to use lacquer, there are some water bourn finishes which are gaining some popularity among some small shop builders. The process is essentially the same, but with waiting times of one day instead of one week. I have never tried them, but the one which gets the most acknowledgements is the one being marketed by
Luthiers Mercantile. You can get all of your polishing supplies, and the fre-cut sand papers there as well. Best of all, with the water bourn finish, you can brush it on, and it is non-toxic, so you don't need a booth or (in theory, though I wouldn't try it) a mask. Water bourn makes it possible to do the finish your self, but I still think it looks a little blue, and it does not have the long term appearance and sound change that you get with lacquer.
Light
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M.K. Gandhi
