I have been using shellac for years, French polishing my pianos of
all types as part of their restoration.  Methylated spirit (alcohol,
methanol and pyridine, the violet colour) is used for thinning shellac
to whatever consistency is required.

50% shellac and 50% methylated spirit is used for sealing in fillers
and colour staining, et cetera.  If bought from a wholesaler, say, in
5 liter amounts, it would be reasonably priced.  Hope to have helped.

My kind regards to you all,
John A J Oakes