John,
I've not done this and what I say here is an intuitive answer, I have no proof I'm right.
Shellac is an alcohol/spirit based product. Once the alcohol/spirit the residue ‘shellac’ is a hard but brittle layer. It works very well on smooth finished surfaces.
But I’d doubt it would form a good seal on part turned work, especially wet timber which ‘will’ move and cause it to crack.