I often see advice to sand between coats of shellac. But exactly how? What grit? Should a lubricant be used? Will the paper clog up if sanded dry? Exactly what am I trying to accomplish? Simply removing brush marks?
I use steel wool 0000 I rub out the imperfections, wipe down with a tack cloth. Then recoat. Mike
Unlike poly, you don't NEED to sand between shellac coats in order to allow good adhesion of subsequent coats. The alcohol (solvent) dissolves a bit of the previous coat, so they actually fuse with each other. Therefore, brush marks on the first few coats shouldn't be an issue in the final finish. If you get dust nibs or raised grain or whatever, you can sand lightly to smooth it out. I only use steel wool (0000) on the final coat when using shellac, or ultrafine sandpaper (400 or higher) works fine too. Keep it light - just enough to make your final coat feel nice and smooth. As long as the shellac is dry, it shouldn't clog too badly. Good luck, Andy (who has learned a lot from Flexner's finishing book - I'd highly recommend it, or Jewitt's, or Taunton's...)
I'd always say "rub down" rather than "sand". It's rarely a big bulk- shifting process. Sometimes for decoupage you might want to shift a lot.
The first thing to remember is that shellac cures by solvent loss and that it's also thermally unstable. The surface dries quickly, underlying layers can be anything but. I often leave shellac for a month before finally polishing it. Black shellac especially so! Also shellac might look firm when cold, but it softens when you start working it.
I'm doing some fake laquerwork at present, with thick black shellac over stingray skin. You paint the knobbly skin until it's smooth and black, then sand back to expose the tops of the skin knobbles (as white circles set in black). Mylands' black shellac permits sanding after a day or two, Liberon's needs a good week before it's workable. Old unstable shellac ruins the piece forever. Really I ought to be using urushiol lacquer...
For French polishing you use pumice, sometimes at the same time as the wet shellac (French style), sometimes between coats (English style). You may even use 0000 wire wool between coats (and let them harden a bit more first). A wipe afterwards with white spirit (mineral spirit?) removes the dust without softening a dry surface.
For final rubbing out, rottenstone (even softer than pumice) is traditional. You can sometimes use powdered charcoal for final glassy polishing of opaque shellac finishes, but it's hard to get the right grade of charcoal. Either ball mill it yourself from home-made willow charcoal or buy real high-end pyro-grade charcoal.
Plastic abrasives (3M / Webrax) are worth experimenting with.
Silicon carbide "wet and dry" paper is useful too, because it's easily available in fine grits. 300 - 2000 are useful here. I've always needed to have a well-dried surface finish before using it though and to work with a water lubricant. Make sure you dry this off well before applying more shellac, or you'll get clouding.
Only _very_ rarely (high-build fake japanning) do I use my usual woodworking yellow aluminium oxide abrasives on shellac. 240 grit goes through it as fast as you might ever want. You might lubricate with water, you might just wash the paper clean with it.
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