Is there a temperature below which it is not recommended to apply urethane on exterior woodwork (door?)
The temperature here is in the mid-50s with about 50% humidity. Obviously it goes much lower overnight, into the 30s.
The can gives drying times for optimal conditions, with 77 degrees. It does not state at which point you'll have the urethane taking too long or simply not drying.
I am using Helmsman Spar Urethane.
My experience is that it dries really fast, normally.