first ide like to say hello becasue this is my first post on this forum
next i have to say that i did a search on this topic however i did not get the specific answer i wanted

so i have about a half of a quart of POR-15 left after i did the engine crossmember and rear axle(what i could get to at least)
and i wanted to do my floorpans. i have an 83 z28 which doesnt seem to be too bad on the floorpans (i think the road grim keeps the moisture out haha) anyway i think their is the factory crap undercoating there already (the grey stuff) its not in bad shape at all just a few places here and there which look to start to peal and get coroded so finally my question is"

can i clean the grime off the best that with marine clean and use the POR on the grey stuff or ... is it a must that i find a way to scrape all of the grey off first? and i know that the por stuff works best on metal contact but i dont have any torches or grinders to get all of the factory stuff off.

i have a few patches where its starting to peel and corode.
is there another way to assist the grey factory paint in keeping the rust away?
thanks alot guys!!

 

Assuming you remove all the loose, flaky stuff and prep it with marine clean, the POR-15 should stick to the factory grey stuff just fine. I've used it over body seam-sealer and it adheres fine to that.