: I used the USComposites thin epoxy (2:1)

I think Rehd is right on the money - the USComposites 2:1 expoxy is SLOW! Essentially, you didn't put on three coats of resin, rather you put on one thick coat. The sunlight & slow cure combo results in air bubbles developing under the cloth, especially where exposed to extra warming.

Here in Phoenix I need to let the USComposites set for at least a day - more if I want to do any scraping to say nothing of sanding. The work you did would have taken me three days minimum with that epoxy.

jrs