Thickness of French Polish finish

[Doug Shaker]

Does anyone have a notion of the actual finish thickness of a French polish finish? Obviously it will vary
depending on how you lay it down, but I'd love to know what it actually comes out as, most of the time.

I would also love to get come idea of how much I am laying down each time I do a pass with the fad.
Seems like hardly anything at all.

[Michael Lewis]

Sounds about right.

You can tell if you are laying down any 'body'. It's not supposed to be like lacquer but much thinner. You CAN build it up but it's a soft finish that way. It's better as a thin finish and applied with pressure.

[Dave Stewart]

As Michael says, the thinner the better. (I believe one of the hallmarks of french polish is unmatched depth & luster with very little deposition.)
Email David Wren, whose gorgeous guitars are all french polished I believe .... he should be able to answer your questions.

[Alan Carruth]

There's no doubt that FP is the thinnest finish. I've measured film thickness in the range of about .0015" when I've scraped off the finish to put on the bridge. That may be generous: you always lose a little wood, which would make the film seem thicker. In the same test on oil varnish I come out around .0025"-.004", depending on the number of coats. Again, this is a difficult measurement to make accurately with common tools. There is an ultrasonic gizmo that get it pretty nicely, but it costs more than my caliper.

I never have gotten the deep luster with FP that I do with a good oil varnish. By comparison the shellac finish seems veiled. It looks better if you flood the surface with oil and wipe it back before applying the shellac, which is the way the furniture guys do it. I avoid oil as much as possible due to the added mass and high damping. Also, in the cases where I have used more oil as a lubricant, I tend to get more darkening and checking of the FP.

When Martin Schleske measured the properties of various finishes he found that shellac was about 1/3 softer than nitro. Gibson ads from a hundred years ago used to decry the 'glassy hardness' of their competitor's shellac finishes as compared with the varnish they used. Nowadays nitro is considered 'soft'. Some of the wear problem with shellac is due to the way it reacts with perspiration in some cases, I'm sure.

[Doug Shaker]

I just wiped three layers of my shellac mix on a piece of metal and found that the three layers increased the thickness by about 0.001 inch. My shellac mix is U-Beaut shellac diluted with three parts acetone to one part U-Beaut.

Advice requested: I ordinarily use a drawer liner that is a kind of pillowy net to cushion my guitar body on the workbench. After I French-polished the back, I let it harden up for a day and started working on the top. I found that after a few hours I had little semi-gloss dots on the back where the pillow things were. I expect I can polish or buff them out, but does anyone have a better procedure for treating the finish while it is hardening up?

[Waddy Thomson]

One way is to keep the neck propped up high enough that only the corner at the tail is resting on anything. Then you can concentrate on that corner to repair any issues. The other way, and what I do is put the neck in a vice and work the body. Hold off on the neck until you can hold the body. One way is to put a clamp, quick grip type in the sound hole, with a caul on the inside, and put the body of the clamp in the vice. That leaves the back of the body and neck available for polishing.

[Alan Carruth]

Some of the foam pads have a plasticizer (silicone?) that leaches into shellac and causes problems, even after it has hardened.

I make a hook out of a coat hanger and hang the guitar up. If you're working the body and neck separately you can hook it through the hole on the box, and a tuner hole or slot on the head. If the body and neck are together, just hang it by the head.

[Michael Lewis]

Use newspaper, carpet, cloth fabric or whatever that is not plastic to put your instrument on. I have some of that drawer liner in my tool box where I keep a mirror, but have to put a cloth over it or it leaves smudges on the mirror.

[Doug Shaker]

Yes, I think the plasticizers in the pad were doing a number on the shellac. Some of the impressions were deep enough that I had a hard time getting them to
disappear and, in the process of removing them, the shellac in that area got too thin. I am now building up again and will re-polish. Frustrating from one point of view, but from another it is a reasonable path for someone just learning how to French polish.

I now have a cord through the bolt holes of the body, forming a loop. Then I have some THICK copper wire (grounding wire for residential electrical service) looped and hung from a ceiling hook. The other end is a simple hook. The guitar body cord is hanging from that. When I take it down to polish more, it will lie on some t-shirt cotton instead of the previous drawer liner pillow.

Live and learn.

[Rick Rosenberg]

I have seen Mario Beauregard use a Finishing product for French polishing which filled the pores and was able to buff to an incredible shine.