Have used POR-15 many times and it seems to stick well only on oil free rusted
surfaces. It will also stick on primed surfaces but as pointed out doesn't stick over paint or surfaces with any dirt on them. The vapors are highly toxic and if it gets on your skin it is VERY difficult to remove. It also deteriorates quickly when exposed to UV. The substance is an iso-cyanate compound similar to Crazy Glue.
I would say that its good to use only over rusty metal and it can be painted over ONLY while it is still tacky.
" THE BLACK PARALYZER" -87GT 3800SC Series III engine, custom ZZP /Frozen Boost Intercooler setup, 3.4" Pulley, Powerlog manifold, Northstar TB, LS1 MAF, 3" Flotech Afterburner Exhaust, Autolite 104's, MSD wires, Custom CAI, 4T65eHD w. custom axles, HP Tuners VCM Suite.
87GT - ALL OUT 3.4L Turbocharged engine, Garrett Hybrid Turbo, MSD ign., modified TH125H
" ON THE LOOSE WITHOUT THE JUICE "