Have had nothing but good results with POR 15 and Wurth (which private labels
it). In every case but 2-3 it was applied over a fresh sandblasted surface. I
used both spray and brush with excellent results and because this stuff is a
HUGE PITA , I was extremely cautious of surface preparation and temperature.
If you spray it and want to top coat later spray on a couple mist coats of
epoxy primer while the POR is still tacky to give "tooth" to the surface for
further paint or primer coats, otherwise it must be sanded again completely.
If you are just dealing with surface rust on the body you might try something
else, but if you are working on rocker panels, floor areas, fuel tanks, the
bottom of window frames, or subframe areas  the stuff was great.