if you're doing a bitch patch, ther is no need to heat the boat or the bitchethane, that may be why your boat got more cracks later. just drill the ends, clean & dry the area, slap that gooey crap on & let it sit in the sun for a while. If you don't have the time to let it sit, then just heat it lightly with a blow dryer till you see it smooth out a touch, then stop. I know folks that have bitch patches & paddled the boat for years after.
heating the plastic is probably the worst thing you can do to certain types of plastic, it just makes it more brittle & become a self fulfilling recurring crack. (had 16 cracks in my last huck, it had 3 & after I had those welded the other 13 popped up in the next 3 weeks) after I slapped some bitch on it I the rest of the season out of it.