higher humidity (70%), or when it’s raining out, is actually worse than lower humidity because it causes the outside to cure faster than the inside. 40-50% considered ideal. I’ve never actually had it NOT cure, even when I’ve painted in winter. So I don't know how low it has to be to actually not cure. In all the time I’ve been using it I’ve just used it without overthinking the humidity. I don't know if the person who had an issue with it used some kind of incompatible reducer in it, or still had cleaning solvent on the metal, or what.
The main issue I encounter is rushing the second coat and getting solvent pop through the second coat. Minimum hour flash, or two hours on a drier day prevents this.
