the only stuff ive used which has 100% stopped rust creeping and coming back is por15 from here

http://www.frost.co.uk/automotive-pa...int-473ml.html

ive painted many things with it and never had rust come back, one of those was the back panel on my mk2 escort while in storage. i bare metalled the backpanel,( tho you can paint this over rust) and painted it, i didnt touch the bottoms of the rear quarters either side of the backpanel which were covered in surface rust. the car then sat for 5 years in my garage at home with a leaky roof and bad damp and the rust never crept to the backpanel not even by a millimetre yet the bottoms of the quarters have now rusted through and need welding

they dont paint the bottom of boats and the stantions holding oil rigs up with this stuff for no good reason!