I'm a little late to the game, but I'd like to add a few things I learned about POR-15 in the last fear years. I have used it in the past, but now I switched to epoxy primers for various reasons.
1. You need to have a "binder" coat to topcoat POR-15, unless you use POR-15's topcoat (expensive)
2. If you don't, POR-15 can bubble up and peel right off the metal it's applied to.
3. It dries extremely hard. Part of being extremely hard is being NOT being extremely flexible.
4. I don't care who makes it or what it is, you want to start with a clean scuffed or etched surface.
5. POR-15 is not UV resistant and will break down with time if exposed to sunlight and not topcoated.
Alright, my reasons for those statements. I have used POR-15 on the floor pan of a 72 VW bug that we had blasted. Worked great.
I had some left over and, after noticing how well drips on concrete last, decided to pour it out onto a cement pad near the entry to our shop. Sure enough, it was hard as a rock after a few days and not coming off without, I thought, a chisel.
A few months later I was painting something and took my leftover mixed epoxy primer and tossed it onto before mentioned cement pad. Thought with enough leftover paint sessions it would be a neat looking chunk of cement, very modern art. Looked pretty cool.
Later yet, I had some mixed basecoat left over. Tossed it on, went to clear, had some leftover clear and went to throw it on the piece of art/concrete pad. However, all the POR-15 had bubbled anywhere the basecoat had touched it. Long story short, POR-15 really does not like UV reducer, which is why you need a binder coat if you want to topcoat it with any urethane product...so most any single stage or bc/cc paint.
After asking around on the forums as to why POR-15 has such a bad wrap in the professional world (you don't see it on high dollar builds unless they are sponsored) I learned a few things. POR-15 dries very hard, because of this when it's on panels or pieces that flex (about everything on a vehicle) it can develop micro-cracks that allow water to seep past. Hard to see with the naked eye but big enough to allow moisture through. Kind of like how something can rust if you paint it with no primer. Also, the very sales pitch and name of POR-15 (Paint Over Rust) is more or less a lie. People had coated old rusty fenders and, years later, scrapped it off to see what's underneath. The rust continued to grow, just under the POR-15. It is also extremely expensive and the formula,extremely cheap to make, is open for taking to any paint manufacture who wants to reproduce their own version, but no one does.. probably a sign?
All that being said, POR-15 will probably work just fine on properly prepared metal. However, epoxy primer will work just fine on properly prepared metal, is flexible, chip resistant, and able to be painted over without requiring extra steps. It's also easy to touch up and much less expensive. It can be thinned without special thinner and sprayed, brushed, or rolled just like POR-15.
Long story short, I'll stick with epoxy primer as it has every good point of POR-15 (except painting over rust, which just plain shouldn't be done) and fewer of the downsides. It is not expensive, requires no binder step, but is still not UV resistant and shouldn't be on external parts without a topcoat.
End Rant.
