I've used both POR-15 and Epoxy Mastic 121. Trouble is, I can't remember which bits I did with which. At the end of one job I cleaned the brush with the supplied thinner then brushed it out over a couple of typically rusty brake drums. No preparation at all, just boingy% thinned paint straight onto rust. About a year later they're still looking pretty good. I reckon it was thinned enough to soak right through while thicker paint would have sat on the surface then flaked off.
Epoxy 121 is the only one that supplies a thinner along with the 2-pack cans, so it was probably that stuff... but then again I might just have used their thinner with POR-15...
Anyway, one of them's damned good!