POR-15 is a gimmick product. Pro's don't use it. Any moisture cure urethene is not what you want to use in automotive restoration. The modern and best way to stop rust is to first remove it with any of a number of ways (dipping, media blast, Ospho or other Phos acid treatments then neutralize) then use a quality (hint not Eastwood DIY garbage) 2 part epoxy primer.
Here's a little thread on what happens to metal that has been treated with POR-15. A good discussion with a lot of Professionals in this thread. You don't have to be a member to view the thread or the pictures.
why you never use POR-15 EVER!!!! | Southern Polyurethanes Forum