Mark Mandell - Gone Round In New Jersey
were you using such long hardener? I do what you're doing all the time and have no issues with 3 hour hardener. I paint the reverse side with Anchor Seal wax and while wet apply plastic food wrap. I put the piece on a flat surface with a layer of paper towels to give a bit of pressure.

Any leaks get controlled within a short distance from the crack and the paper and plastic come right off.