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John or Brian have you ever used sandarac with shellac for french polishing? I've been dinking around with it and not up to putting it on a guitar yet. It's supposed to make the finish as hard as lacquer and even more clear and with chatoyancy. I had to heat it to get it to bind together and it still left a goo at the bottom of the bottle and took a week to distill out. I've used it on boards and it does look nice. What a great thing if the fp could be as hard and protective as lacquer.
Steve Marchione and several violin finishers swear by it.

I never used it. Sorry I can't be of more help

Adding too much Sandarac will lead to crazing of the shellac. It will make it harder, and much less flexible too. I never add any to my shellac for FP, But I have used it in different varnish recipes over the years. If you want a FP as hard as lacquer, use buttons. The process by which buttons are made polymerizes the shellac and makes it much harder and more durable than processed flakes. Button shellac was the finish on ballroom floors prior to the invention of modern poly-urethane type finishes.