POR 15 holds my trucks together, I have used all of their products and they are all nice products. POR 15 does require proper surface prep, you can paint directly over rust and it'll be ok, a better approach is to use their 'metal ready' product which coats/converts the rust to something that looks like its been galvanized. This is the ideal surface for the POR15 paint.
Once its been painted nothing likes to stick to it, if you need to put anything on top of it, including some liners, you will need to prime it with their 'self etching' primer. Once this primer is dry you can put regular paint products over it.
As was said above its not UV stable but its only cosmetic, the integrity of the coating is affected by UV. In strong UV it ages very quickly and looks very dull, most go to a really dull black, the silver goes to a dull gray which is almost primer colored.
They also have a chassis black paint which is UV stable and is somewhat rust resistant, its much better than normal paint but not as good as the POR 15. If you are ultimately going to line it you may be better off doing the metal ready step followed directly by the chassis paint.
Their own chassis paint does not stick to dried POR 15 without the etching primer - ask me how I know...
Like I said I have used lots of their stuff on my 30+ years old trucks (one of which was a snow blower for 28 years), I am more than happy to share my experiences, what worked and what did not.
You don't inherit the world from your parents, you borrow it from your children.
1979 Unimog 416 Expedition Camper
1974 Unimog 421
2004 Dodge Ram 2500, 4x4, Double Cab, Cummins Turbo Diesel
2006 25' Airstream International CCD
2009 Harley Davidson
Sugarloaf, Boulder, CO