You would be better off buying a heat lamp & make sure the metal is warm before applying filler. Just don't heat it too much. I remember when I worked in body shops in Maine in the winter & it would be too cold some mornings to use filler without some form of heat. I tried using a torch to warm it up once.
It worked, but not too well. Too uneven & too hot.
You need a source of even, low heat.
If you warm the filler & not the metal it isn't going to bond to the metal.& will come off in chunks when you sand it.
