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Monthly Archives: June 2008

Wrong diagnosis of the problem with the French polish

It looks as if  I’m wrong about the problem with non-hardening shellac that I mentioned in my last post. There, I suggested that it might have been due to using mineral oil to stop the pad sticking when applying the shellac. Bob Flexner emailed me saying that neither he nor the hundreds of people that he has taught have ever encountered any trouble with mineral oil. Indeed, the reason that he prefers mineral oil to linseed oil is that unless one is careful in removing all the linseed oil, it cures soft and gummy on the surface.

I’d like to get to the bottom of the matter, not least because I don’t want to run into the same trouble in future. So I’ve carried out some experiments to try to reproduce the problem using various types of shellac in combination with various types of oil to see if they harden differently. If anything useful comes of them, I’ll write about it.

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French polishing two guitars

A few weeks ago, I started French polishing a cedar-topped guitar and, to avoid the temptation of rushing to get it finished, I began making another to give myself something to do while waiting for the polish to dry and harden. Absurdly, the strategy worked too well; I became so absorbed in making the second instrument that I didn’t pay enough attention to the one I was supposed to be polishing. This meant that I didn’t notice a problem: the shellac that I was putting on wasn’t hardening properly.

It took me a while both to identify the problem and to come up with a diagnosis. I’d been using light mineral oil on the pad (instead of the usual linseed oil) to stop any sticking as the polish was rubbed on and I reckon that some of this oil had got incorporated into the finish and slowed up the hardening. The advice to use mineral oil comes from the chapter on shellac in Bob Flexner’s book Understanding Wood Finishing, which is otherwise a mine of good sense. It’s possible of course, that I’m wrong in laying the blame at his door but when I eventually bit the bullet, wiped off the non-hardening shellac and started all over again using linseed oil, the problem didn’t recur. I’d be most interested to know if anyone else has had the same experience. Sometime I must make some experiments with different oils to find out whether this is the correct explanation.

All of this inevitably slowed down completion of both instruments and there hasn’t been much to photograph or write about, which is why there haven’t been any posts for the last couple of weeks. Anyway, I’m now at the final stages. Here’s the current state of the two guitars:

Stringing it up

Well, here it is all strung up. Too early, of course, to make a judgment about how it’s going to sound. But it’s nice to find that it works and that the frets are in the right place. I’ve given the back a wipe of shellac to bring out the figure.

The guitar you can see hanging up in the background was the last one I made and which is being French polished at the moment. It has been rather neglected as I’ve been making the latest instrument, but now I’ll get on and finish them both off.

Bridge

The bridge is made out of Macassar ebony inlaid, on the tie-block, with a flash of laburnum to echo the rosette. Earlier today, I positioned it on the soundboard and glued it into place. The clamps are now off but I’m going to be patient and wait until tomorrow before stringing up the instrument. It’s always wise to let the glue cure completely before putting a lot of tension on the bridge.

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