POR15 will seal in the rust. If flakey, scrape and wire brush it. I like to remove all I can, then apply the POR15. It is very susceptible to UV deterioration however, so you will have to overcoat it with regular spray paint. Buy the cheap "chip" brushes, throw them away when you are thru for the day. Do not get any in the groove of the can where the lid seals. If you do you will never get the lid off again without virtually destroying the can. Wear gloves, and an old long sleeve shirt, or simply a painters tyvek coveralls. It takes weeks to wear off your skin, and you will never get it off anything else you get it on. Use large cardboard boxes opened up flat in the area you are painting in, so you don't get any on the driveway, etc.
I just used it today. The rails where the battery box was bolted down was rusted and pitted and I sanded it clean and applied POR15. After it cured, I sprayed on a coat of black "chassis paint" from Summit Racing (had it left over from another project). Scrubbed the battery box clean inside and out, and installed it with new hardware.
Charles
'03 Ram 2500 CTD, 5.9HO six speed, PacBrake Exh Brake, std cab, long bed, Leer top and 2008 Bigfoot 25B21RB.. previously (both gone) 2008 Thor/Dutchman Freedom Spirit 180 & 2007 Winnebago View 23H Motorhome.