Just want to add a note here because this thread is one of the first to come up in searches related to epoxy not curing or kicking as expected:
I discovered “a new one” today when it comes to epoxy refusing to cure or kick properly. Setting Proboxes, I used a different modeling clay than I normally use to help seal off the boxes and screw holes from epoxy leakage, and discovered, a couple of hours after pouring my RR Kwik Kick, that I had a weird mix of cured and uncured epoxy.
Normally, the prime suspect would probably be an improper mix, or incomplete stir, but I’ve never had this happen before, with any resin, including the 5-7 times I’ve used KK, now.
I have no way to be sure, but it seems likely that the culprit was the change in modeling clays, as I noticed that anywhere resin clearly had run over the clay, or been exposed to the clay, the resin didn’t kick, while the same batch cured fine in other places where it clearly hadn’t touched clay.
I thought this originally, but didn’t believe it after reading about so many other potential causes, but reading around some other web info I did discover that clay (specifically containing sulfur) can cause “cure inhibition” in rubber (http://www.smooth-on.com/faq_display.php?faq_id=4).
I don’t know that the clay caused my still ongoing issue, but it seems very possible, given that I’ve never had this problem before in building 12 boards with epoxy (and a couple more with poly).
At a minimum, my mishap’s a good reminder that “if it ain’t broke, don’t fix it” when it comes to glassing recipes. I should have just paid more for the modeling clay I’ve used successfully in the past, rather than try a new one because it was cheaper.