It's called 'paint-over-rust" for a reason as it needs grease/oil free texture to key into else it might/can peel off like the shrink wrap on a box of chocolates.
POR-15 is a really thin paint (a little will go a long way) and the thin body carries it deep into the surface pores. It has some fierce carrier solvents that then evaporate to allow the real cure to kick in that gives it the hard-as-glass finish. Using a wire brush then the 3M-style abrasive pad biscuits should have rusted areas ready to paint - it's the unrusted areas that cause problems.
I gotta note here the frame paint on my 72/73 trailers is/was more a waxy creosote-asphalt undercoat than paint judging by the throw-off from the wire brush - and even bright metal surfaces ran yellow/brown when hit with carb cleaner spray from the residues left behind (I used about eight cans of Chemtool B-12) Originally I tried not to shell-off my trailer so did my frame twice, once with floor-on and the second time with shell-off, the areas slathered with POR over decent original paint from the first time would peel away in large flakes when the wire brush hit it. I'd guess the POR-15 carrier solvents melted some of the OEM paint altering the finished product, painting only the rusted areas and letting the catalysts kick then overpainting the entire area would be a worthy step.
However there was no/low new corrosion on overcoated areas compared to the inside ladder frame (protected?) areas I simply wiped down and sprayed with Rustoleum.
The Metal Prep product precipitates zinc crystals onto smooth bare iron to give POR-15 something to bite to, done correctly it leaves the surface resembling 1000-grit sandpaper. If you get enthusiastic and have a bunch of bare unrusted metal its the way to go.
Beware of the single-step rust conversion paints - I've seen the plastic finish result be completely undercut leaving one big blister of shiny black paint on my truck differential cover, to the point it almost perforated in just a few years.
Ah, so... Here is a link to my latest resto thread
for your amusement.