One last time, Larry . . . ;-)

My surfboard buddy, Dan,stopped by the house to check progress this evening, and I related your predicament to him. He said of all the board mfrs he's worked for up & down the caost, he'd never seen the problem as you describe. They would not be able to build the boards the way they do if sanded glass didn't disappear in a fresh coat of epoxy. They sand with abandon, and never worry about fill coats before sanding. This is my experience also.

He said perhaps contamination of the cloth or poor coating on the cloth, otherwise it would wet out and disappear. I will add one other possibility that came to mind - Dull sandpaper. If instead of cutting the cloth, you simply rubbed the coating from the protruding glass strands, it will never again wet out properly.

FWIW, Spidey