Henk,

I agree with John. A bit of air probably escaped through the cap during a hot day. Then, with cooler weather, the air volume inside reduced, causing the sides to cave in a bit. If the hardener (liquid) had vaporized, the volume of gas in the container would be much greater, causing the container to expand. I don't know about epoxy hardener, but water expands 1,700 times when it goes from liquid to vapor.

I'd follow John's advise; mix up a bit of epoxy and try a small test panel. It'll probably react and harden just fine.

Hope this helps,

Pedro Almeida