Why You Can’t French Polish with Polyurethane
I got a question from a woodworker about French polishing. He wanted to know why he was having so much trouble French polishing with Wipe-on Poly. He had made his pad as instructed, putting a wad of cheesecloth inside a cotton cloth. But the pad was taking off as much finish as it was applying, so he wasn’t getting any build. He wasn’t making any progress.
The answer is that French polishing only works with shellac. It surely doesn’t work with a varnish (wipe-on poly) or water-based finish. He might be able to figure out a way to do the technique with lacquer, but I think it would be very difficult to avoid serious streaking and drag marks.
Shellac works because it dries very fast (like lacquer also does). So every time he pads back over a previous stroke, he deposits a little more shellac to increase the build. In contrast, the varnish remains wet, so the pad is just moving it around. There’s no build because there’s no drying.
Shellac works better than lacquer because you can add a little oil to the pad to create a cushion than helps to avoid streaks and dragging of the finish. The technique of French polishing was developed in the very early 1800s when the only film-building finish available was shellac. Oil and wax, which existed then, don’t build a functional film.
– Bob Flexner