the purpose of glaze is to fill major sand scratches ex. 80 grit. glaze is a fine filler that is easly sandable with a higher grit paper 120, 180 220 grit it also fills pinholes....finish with no higher than a 220 grit paper, then prime!
I agree, that glaze you are talking about is probably laquer based and is real proned to shrinking, I use evercoat metal glaze at work and I really like the stuff.