Polyester resins absorb water, and do not surface cure. You can control the cure time of polyesters, within a range, to compensate for temperature, to a limit.
No need to heat the whole garage, you can also use a radiant heat source on the panel. Infrared lamps, or something similar.
It is just going to take more work to sand. If you can get a good feather edge, you are probably good to go.
Because polyester resins do absorb water from the air, I have decided it is necessary to put a good epoxy primer over the metal before using polyester filler. I believe the polyester can absorb enough water to start metal to rust under it in some conditions.