"Ask any paperhanger who has left an open five-gallon pail of your paste in a hot truck for a week."
Typical hyperbole. Then again, you didn't ask this paperhanger, or anyone I've worked with (and that would be dozens of paperhangers). I've kept 'open'- that is, a pail whose cover has been removed, but replaced on storage- cans of paste for months. What I don't do, though, is put paste that has been used on a job back into the fresh paste- that introduces contaminants that lead to mildew. Paperhanging 101.