Paint the frame on a Non-frame off rebuild ?

Wire brush, wire brush on a right angle grinder, tooth brush size wire brush, brake and parts cleaner to remove oil and grime and a couple of cans of shake~N~shoot. LOL
Of course, over here, we do not have a problem with the liquid car remover like in some other states so rust is not much of a problem, unless Your working on one of the DOTs bin sanders. LOL

the POR-15 system is a solid, well-reviewed paint/rust-inhibitor. That's what I used (on my frame-off rebuild).

I used POR-15,and followed the instructions carefully. It is a very messy process, it has been almost three years since I used it. I'd say it looks pretty good considering that we live on Cape Cod where salt is always in the air, causing corrosion issues. We did put 8000 miles on the camper last year...lots of bumps and bruise and only the very front of the frame was damaged. I think that POR used anywhere away from salt air would be a very good choice. (Not sure what might have been a better choice here?) As mrmarty51 says, prep is most important. Also, knowing that we have this problem, I made the decision early on not to try polishing anything. I try to wash and wax regularly to minimize the damage to the aluminum trim, but I'm afraid I may be losing the battle. Sort of the same issued regarding paint. The side facing the sun is faded and chalky with streaks from aluminum trim, while the side away from direct sun looks like new.

i used por15 as well. note that it is not uv resistant so it will need top coating. even though its under the trailer i top coated my por15 with a rustoleum just for the uv protection. i have no idea though of actually how much uv reflects off other objects.

 before: 



with por15 on the frame and a rustoleum primer on the belly pan:




top coated everything with rustoleum gloss black:





this is the por15 after drying. it lays out smooth even with a brush.