If you have a ceiling you could always put in a Whole House Fan. Leave the garage door up about 3-4 inches and vent it through the ceiling. It would also help keep the roof cool too.
Pictures would help everyone give you ideals. Some taken at daytime and nighttime.
I've often considered installing a whole house fan in my garage, leaving the back screen door open. However, wouldn't it be risky to use such a fan when working with volatile solvents, etc.? My concern is what would happen if the attic space was filled with volatile vapor.

