I've never tried any method for renewing the shellac, so I'm just repeating what I've read on this forum for the last several years.
Glenn, who used to frequent this site, wrote a tutorial for renewing the shellac on machines.
It's in the stickies at the top of this forum. If you post a question on that thread, he will reply. He frequents
victoriansweatshop.com now.
I've been advised by the "old guys" to use auto polish. There are specific brands that are recommended. I have TR3 resin glaze, which I believe was a recommended polish. But, as I said, I didn't try it.
I think a lot depends on how bad the shellac is compromised. Water will silver the decals.
bkay.