I tore my new mustang apart in january with 300 miles on it, put a gallon of bedliner under it- pulled fenders, rocker covers...that stuff is pretty neat, I think It will last too, as its kinda made for abuse...neat thing is you can 'cake' it on very thick and it dont seem to shrink up or crack, but is easy to smooth/fillet into corners...
the paint I used on the gal was POR-15. you gotta use a metalprep solution(mild acid) to neutralize the rust, and etch the surface before using, but after doing that it just about cant be removed. bad thing is its very shiny/slick, so paint wont really stick to it(they sell that blue primer to put over it if you want to overcoat) and I used some under the mustang over paint, and it just blew right off soon as it dried...only works on bare treated steel. After using the bedliner, I have a new favorite- I would still suggest acid etch first to stop any existing rust from starting underneath, then just use the bedliner. one other thing- bedliner is UV stable(would have to be for bed use) where POR-15 is not- it bleaches out kinda grey/white if out in the sun much. POR will stop rust, but it is the messiest stuff Ive ever touched...bedliner should hold up nearly as well and seems to stick to anything...just scuff it a bit. I did inject por15 inside my framerails on the mustang till it drippied thru the gaps in the pinchwelds- where the gaps are filled, I'll still have tapelike por15 loose inside the bigger areas...dont even try using por15 over painted ares- it just curls up as it dries.
I shoulda just used paint on the new car, as theres basically no bare steel. the driveshaft/rear axle are bare and well rusted already, will probably POR these as it leaves a very nice durable shiny surface unlike the textured of the bedliner, but heck- its underneath anyways...maybe I'll just etch/bedliner them too...
for etch, around here ACE hardware sells "Ospho" brand phosphoric acid metalprep- you could also use duro 'naval jelly'- its about the same thing...ospho you can spray in a cheep spray bottle, just watch out for mist- dont breathe or let it settle on stuff it could mess up...I usually take the stuff outside, spray the grass down so any mist wont settle on dry grass and kill it, spray the dry part down with acid(stay upwind!) let sit 10-15 mins and wash it off with lots of water...lots and lots of water...then more water, then let it dry really well- if rinsed well enough, any rusted ares will just be kinda dark, but if not rinsed enough the part will look white/dusty from residue...but the stuff Ive painted after treating has never blistered...my 85 camaro was my first attempt at car painting, and it was done 11 years ago, all old surface rust treated ares still are perfect...the stuff does work. I like the bedliner for undercoating. heres a pic of my naked mustang:
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I filled the inside/bottoms of my doors with bedliner, blended smooth to drains- no water will get into crimps...also masked/bedlinered the underside folds of same crimp to keep water out too- every car Ive had got first rust in bottom door crimp or rocker lower pinchweld areas. Also if youve got any seams to fill under your floorpan, 3M 'fast and firm' sealer comes in caulking gun tubes, and its good stuff- just dont put on too thick as it shrinks/cracks if over 1/8 thick...just use a little and overcoat with bedliner. thats my 2 cents on it anyway...just remember- por-15 over paint dont work at all- trust me...instructions say similar, but as hard as it was to get off everything, assumed it would still be fine-I'll be peeling off whatever left next time I get it up in the air...the POR under bens car aint going nowhere though- over etched steel, that stuff is hard as rock.
