That is a tale of woe. We have probably had one to one degree or another.

If you're not too frustrated, MAS will be the final word on what to do. In the meantime, black plastic in the sun won't hurt anything if you've got the time. I'm going to tell you that I had a small project that I was using small batches for and I got the ratio wrong. It was still rubbery (it had graphite in it) after a week, so I got frustrated and set the project aside. I was cleaning the garage a couple of months later and I ran across the part, which had cured rock solid. It sanded nicely, and I got all excited and finished the project. It was not a kayak and I did not have that many hours in, but maybe it will make you feel a bit better.

m