POR is some awesome stuff. Just remove the loose stuff and put it on... FYI wear gloves/long sleeves or it'll be on your hands for a few weeks

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I would think following any automotive painting technique would work just fine. Make sure whatever paint you use has UV blocking in it. Rust-Oleum or something like that does not and the paint fades fast and gets chalky.

Personally I'm not a fan of POR. I've used it on new metal, old metal...rusty metal and clean metal. I couldn't get it to stick for any length of time. Six months in it would be peeling. That's just my experience. I'd rather use an epoxy paint or even an enamel paint with hardener.

 

POR 15 is awesome stuff when you follow the directions and prep thins right, DON"T do that and you won;t be happy
and its NOT really a PAINT< its a rust stopper, and has to be top coated for it to work, if you DON"T top coat it, it will fail

as for it sticking to things, I have never had it NOT stick to things, as a fact, I have used it as GLUE to hold heavy gauge wire mesh covers in place on heavy equipment, has metal mesh over radiator covers and such that used to rattle so much it drove me nuts running the equipment, a few drops of POR 15 at all loose spots,. and 7 yrs later things still locked on like a weld?
I have also used to to fix pin holes in radiators, so

LAST example LOL
I had a piece of 6x6 treated wood I spilled a little on and didn;t notice, I placed it on my work bench,
came back next day tried to remove it and ???
it had glued itself to my work bench so much so, I , needed a sledge hammer and some cursing to to get it off bench?

BUT as for painting your trailer, like all things, the prep matters a LOT then use any good paint meant for the type of metal your using and or primers first !
if you do a crappy prep job, it will peel and come off,/fade!

 

With the amount of time and money it would take to correctly paint your trailer you might find it easier to just re skin.

 

I would guess painting a trailer is the same as painting a vehicle, unless your one of those guys that uses a paint tray, and a roller.

 

I had a buddy in h.s that lost his license...we lived in a small town so everyone knew everyone...he would paint his Monte Carlo a different color primer ever couple of weeks. He got away with it for quite awhile before the LEO'S caught on.