I used it on a chevelle chassis, the por that was on there was flaking off. I stripped it down with a wire wheel and sprayed a few coats of the eastwood encapsulater. Looks really nice and dull black, I don't like the gloss por has for chassis parts. As for holding up, I can't say. I shipped the car last December. But I would use it again.
Permatex also makes an rust converter
aerosol or brush on can be painted over
you can get it locally at your favorite parts store
I used this with a spray nozzle on it , to get inside the rear frame rails , when I had the tail panel off
Eastwoods RE is pretty forgiving. I'd scuff the surface, then use a prep all, then hit it with the RE. Give it a medium coat for the first application, then wait one hour and hit it with a heavy coat. Let it dry a solid 24 hours before mess in with it. It flashes off pretty quick but stays soft underneath for a full day.