I use Duplicolor filler primer while working on pieces for its fill qualities for small sanding scratches and as a temporary cover. When I am ready to paint I use the sealer/primer that goes with the paint I'm going to use. I don't have a lot of experience painting, but I did learn that whatever sealer/primer you use to cover the filler primer needs to interact well with the filler primer so you don't have to sand it completely down again. With the paint I used, you only have so much time to paint the base over the sealer/primer (without re-sanding). So, you need to just about to be ready to paint once the sealer/primer is put down to be efficient. Also, my truck is in the workshop, and is not exposed the elements while the filler primer is on.
