If you are going to use POR-15 on bare metal, there is a etching compound that should be used first. the etching stuff makes the POR-15 think it is being applied on rust and sticks better.
I have only used the high temp grey and aluminum, both sets of headers were sand blasted, wiped with the etching stuff and wiped per the instructions with a wet rag and then the POR-15 was applied. Both sets of headers were baked in a Power Coater's oven for four hours.
The grey ones still look great after 8 years of be usd in the NE winters in a Ranger/5.0L setup. The alumnum ones still look brand new because I havn't used them yet but will test them out soon.
Good Luck, the POR-15 is expensive but it is worth it.
Do it right the first time !