What Grump said - Shellac is a great seal coat for nitro so you certainly don't have to sand it all off. Any oil that may be on or in the shellac should have been removed by spiriting off but if that step was not completely done you can still spirit off now prior to spraying nitro. Using naphtha is just another way to remove any oil residue too and could also be called spiriting off me thinks.
I have to send Grumpy an email - we compare pain in the winters by me telling him it's freezing here and he one-ups me and tells me it's a LOT colder where he lives - this is getting to be our winter tradition....
