I''ve never use polyrethane for glassing, but I''ll put this out for discussion. The stuff doesnt really dry hard for a few days or longer. Here in California, with all our clean air laws, it may take a few weeks for our mixtures to dry hard. To be safe, I would try a test piece and the see how it holds up to the heat gun or iron when putting the covering over it. I would think that trying to cover it to quickly would result in some of the solevents in the Polyurethane trying to steam out and cause bubling in the glass and covering.

Don