I'm a furniture restorer who's just completed an oil painting. I was thinking of trying French Polish as a 'varnish'
I had to look up French polish to understand what you were talking about. Shellac was a common varnish material in years past but it isn't much used anymore. There shouldn't be any issue with using shellac that I'm aware of. However, it looks like French polish is more of a process than a specific material and that could present some problems with the variability of materials used in the process. What I read said that alcohol might be used and I would highly recommend against that. Alcohol is a great solvent and could damage the paint film. Plus, being so volatile it could produce a brittle varnish surface that cracks as the oil paint film underneath completes it's cure. In addition, the use of hard drying oils in the process will probably lead to a varnish film that is as hard and difficult to remove as the paint film itself (possibly even crosslinked into the paint film!) and this is definitely NOT a good quality for a varnish for a painting. The use of abrasives is also very concerning. Whether or not that's a legitimate concern will depend on how you paint: thin glazes or thick, does any texture integrate into the composition, etc. I think while the purpose in varnish for furniture and paintings is similar, it also differs much due to surface the varnish is protecting. I'm sure you're aware that it's important that varnish be removable and for a painting, easily so. On furniture, you can use powerful solvents and abrasives to remove that old weathered varnish, but not on a painting. The varnish should not integrate itself to the paint film and removal should in no way endanger the paint film.
I've never tried this varnish method so I'm not fully qualified to say it's a very bad idea or a great idea but some of the things I see cause me to be concerned. Personally, I'm a firm believer in varnishing my paintings. Not everyone does, some like a flat, unvarnished finish. I like a light glossy finish. Too glossy and the painting does not show well, the actual paint can get washed out in the bright glare. Just a light amount of gloss brings up the brightness and makes it glow. A simple, thin brush on application is all it takes to make me happy.
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