throw out your 5-6 minute epoxy. It is crap. ALWAYS use at least 30 minute epoxy. Fast epoxy is weak, turns brittle, crumbles off, is heavy and a waste of money. If it needs epoxy at all, it needs good epoxy.

Also, I think thinning epoxy is a really bad idea. Many don't agree with me, but many do. Heat works great. Put it in a pan of warm water and in a minute or two it's plenty runny without messing with the chemical reaction.