POR 15 is a variant of marine barge paint. Super duty layers of protection. The basic material is aerobic. it cures with oxygen, and not as a result of drying solvent. Any trapped air under the coat will be absorbed by the POR stuff. Therefore if you prep properly, pretreat with zinc and apply POR 15 completely, no oxygen will penetrate the POR coating (unless its scratched)and no iron will react with oxygen to make rust.

I have used this stuff for 10 years with great success. Even used it as a primer for 4 sets of outside doors on my house. With 10 years of exposure to constant Florida humidity and dew, I have one line of rust on one door as a result of my missing an overlap of coat. The doors were already 8 years old when I did this job and were pocmarked with rust dimples and spots. I took down the doors and sanded down to bare metal.

The POR has a hard surface. The toughest part of my door job was getting the acrylic paint to stick. It would literally slide of the Por primed doors.

Great Stuff and it is really expensive. The key is to get the metal absolutely rust free.