My discovery is that it's air getting into the can. I've found I can buy a larger can, not much more expensive than the little can, then fill an empty little can from it. The little can contents are shot after a few uses, but what's left in the big can is good as new. Another find with contact cement: When I buy the smaller glass bottles, it's dried up in a few months after first opening it. I bought one of the cans, about a half pint in a small metal paint can. After a few years, and many openings, it's still in good shape. It's maybe 10% used up by now. I would have thrown away at least a half dozen hardly used glass bottles of it over the same time.
