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wrote:
When polyester resin is past its shelf life what happens?

My supplier of surfboard resin warned me about the shelf life of the
typical slighly greenish surfboard resin. Also, I have read that
catalyst or MEKP has an even short lifespan.

I have had a can of resin and some catalyst stored in my upper Midwest
backyard shed since last summer. It went through a winter with many
subfreezing days. I have read that once a can of resin is in freezing
cold that it will never be the same.

The stuff looks fine. Yesterday, I made a batch. It cured normally.
I have a 3/8" thick disk now. I cannot break it with my hands.

All I have to say is: huh?

Polyester resin will eventually cure even if it's uncatalyzed, but as
you've seen, it can last a year or more at times. I've got some that's
at least that old and it still works fine. Keeping it cool extends it's
shelf life, but I've seen warnings against freezing on the cans.