shiny new parts....

I'd use a little more thinner at those temps and give it longer to dry. If you can get some cheap infrared lamps that would help it along if its inside. Direct sunlight will also help a lot. 50 degrees might be 90 on the surface.

And the final dry time when your ready to start finishing - I'd give that as long as you possibly can.

Your other option is to add some hardener. This thread has some info on that..

http://www.rolledon.com/news-suggestions-f7/rustoleum-advice-t367-25.htm#3394