it actually adheres best to rust, the instructions tell you to get the loose rust off and clean the surface to be sure its clear of dirt and grease and you can brush, roll or spray it on. i brushed mine on my frame. one quart should do an entire frame.
any automotive paint supply store will have it. ive also seen it on ebay and other online stores. it costs about $40 a quart. just a word of warning, DO NOT get it on your hands, it really does take weeks to get off your skin. once the stuff cures onto the frame if its prepped right nothing short of a grinder or chisel will get it off, its tough stuff.
http://www.por15.com/
heres a little before and after from when i did my frame and suspension