Yeah I get a copper pipe from hardware store and beat it flat with a hammer and stick that behind what I am welding. It acts like a heat sink to keep the metal a little cooler. And weld does not stick to it. Really helps a lot.
Those door gaps look great. I like to let the high build primer set for a week and then block. Gives it time to settle. That way when you paint it sand marks don't show up later on.