If I have rust metal I wire brush as much of the rust off that I can . I then apply Ospho, wait 2 days,remove the blk residue & prime w/PPG's epoxy primer. All the body work is then done.

A lot of people use POR-15 but, I look at it as a quick fix....not the best fix. Here is a Body/paint site that you might want to visit.
WWW.bodyshopsupplies.com

Don