constructordeguitarras -> RE: French Polishing (Sep. 4 2012 21:20:49)
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I want to share more about french polishing, but I can't do it all in one posting. By the way, Do the padded vise jaws you are using not mar the finish on the neck of the guitar you are finishing? I use about a one pound cut for everything. The seal or spit coat can be brushed on, but then it will raise the grain. I find that if it is padded on--which I do without oil to avoid getting oil into the wood--then I can just keep going with no sanding necessary right after it. For pads I have tried many things--T-shirt fabric with wool or cotton wool inside, linen outer layer, silk outer layer--and I have found that a simple wad of #50 cheese cloth works best. As I've mentioned, I use olive oil and find that the oil is automatically removed at the end when I use automobile abrasive polish. After sealing everything, I fill the pores in the neck using pumice and shellac without oil. Then I sand the neck lightly with 400 grit. I polish the back, maybe for 30 to 60 minutes. Then I do the sides by standing the guitar on end and holding it by the neck with my left hand, polishing with my right between the heel and the widest part of the lower bout. I also polish the heel in this step. I do this, switching back and forth between each side (rib) for maybe 30 minutes. Mostly I use small circular strokes, occasionally doing long "straight" strokes on the 4 long edges (corners). Next, I lay the guitar on its belly and do the tail, between the tail and the widest parts of the lower bout, with my left hand on the back holding the guitar down. Now I turn the guitar around and do the neck and the back of the head for maybe 30 minutes, holding the guitar by the sides of the head for the most part, sometimes leaning on the back, with the belly down. Then I hang it up to dry for at least several hours. After the back is dry, I do the soundboard, with the bridge in place. I sealed the bridge before I glued it on, but it still needs more polishing. I have no problem polishing around and over the bridge, and up to the edges of the fingerboard, using a small pad, maybe an inch in diameter--but they flatten out nicely like a pancake if there is not too much fabric, and this makes it easy to run up to the edges of the bridge and fingerboard. Sometimes I make "straight" strokes from the bottom edge of the soundhole, around the soundhole and along the fingerboard edge, to the top of the soundboard, which assures that shellac gets on the fingerboard edge from the edge of the pad. And sometimes I make long strokes around the corner between the sides and soundboard. The long strokes make sure there is finish on the corner and also that there are not drips or globs built up there. I work the soundboard for 30 to 60 minutes. Then I do the face of the head and the sides and crown of the head, alternating between each of these areas for about 15 minutes. I let the guitar dry overnight. When dry the next day, the finish may appear to have tiny bumps in it. This may be due to bits of the fabric from the pad wearing off and getting into the finish. If present, I sand them lightly with 600 grit using oil as a lubricant. I have found that whenever sanding shellac it is important to use oil as a lubricant. This keeps the powder that is formed from sticking to the surface and making a cloudy appearance. Sometimes naphtha (mineral spirits, paint thinner) has made this worse, although it seemed like a good idea because it is much thinner than olive oil. The next day I do the same thing and keep going on in this way until enough shellac has been applied, usually a week or two. Because the shellac continues to shrink for a long time it is easy to be fooled into thinking that there is enough when there isn't. It is very important to let it dry enough, 2-3 days at least, before the final buffing or sanding/buffing. Depending on what kind of a finish you want you could level with 600, 1000, 1200, 1500, and 2000 grit sandpapers and then buff with automotive abrasive "scratch remover" polishing compounds, or you could use 1200 grit and then apply a bit more shellac and then buff, like Andy does. Sometimes I like some texture to my finish--it shows that it is hand made. Many people want a perfect finish that looks like it was sprayed on. It's kind of neat that if there is a texture, it shrinks as the shellac shrinks over time and looks more and more refined. A few spots will have to be finished by brush: some inside corner details on the crown of the head, the inside corner between the heel and the side (sometimes), and the valley of the bridge.
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