If its still gummy, leave it for another week or so and see if it dries. If not, you'll need to remove it.
If you shoot over it you will just be trapping the wet epoxy under and you don't want that.
20 degrees at night is a huge swing in temp. Do you have a space heater for your garage?
Epoxy is great stuff, it's too bad you are having problems. I feel for you.
I remember spraying my axles with epoxy and it was terribly humid here in Florida and it took a solid two weeks before it dried to concrete.
