I'm in the process of finishing off the trunk area of the Nash. There is some surface rust that I just want to apply either POR15 or undercoating. My thinking here is that the undercoating would provide a better surface so things don't slide around in the trunk. Can you apply undercoating over surface rust and it will seal? Or will it just peel off later. Can I apply undercoating over POR15? I could POR15 it first then apply undercoating I guess...
Results vary with POR. Where possible/practical it's always better to clean to bare metal. Best option would be to media blast or mechanically/manually clean, treat with phosphoric acid(don't breath too much of the fumes), then prime and paint with good materials. Epoxy and/or etching primer are good(don't breath those fumes either).
Like said before, try and neutralize the rust first. I know i've seen undercoating with a rust inhibitor in it, but I can't remember the friggin name of the product now...
The undercoating will peel if it's applied over rust. I would sand, apply a couple coats of rust reformer and then spray on the undercoating.
What they said. Covering rust doesn't do anything except allow you to forget about it until it works its way somewhere else. -Dave
I was thinking that it would be a good surface so things won't slide around. I don't want to put carpet back there. Maybe I should be thinking of using bedliner instead?
Go with the bedliner. Hot weather will soften up the undercoat and make a big mess on anything you keep back there.
Use the POR15, and follow their directions, wire brush all the loose surface rust away, and you can put undercoat or bed liner over it, it will neutralize the rust and bond to the metal. Though like 1971BB427 said the undercoat will stink.
Exactly zman, I used por-15, 6 years ago on my 55 and I used it inside , underneath and anywhere else I could get to and it's never flaked or chipped off. Great stuff but unless you use a resperator DO NOT SPRAY IT. Aiborne it's lethal crap that burns the crap out of your lungs. ( I found out the hard way) Use a foam brush and alittle goes a long ways
Don't use undercoating, the rust will keep festering away underneath. And, anything you put in the trunk will get covered with tar. Paint it with POR15, then some kind of finish paint, Zolatone is traditional for trunks. http://www.zolatone.com/stories/the-zolatone-story/
The rusting process requires oxygen/air. If oxygen/air is sealed off from existing rust, no additional rusting will occur. But..... as was already mentioned, the problem is; rust does not provide a good surface for coatings to adhere to.
POR15 over the rust (follow the directions!) then just as it flashes and is tacky (but more towards the dry side) spray it with two part epoxy bedliner. It'll be tough as nails. Most coatings don't like to stick to fully cured POR15 and the bedliner I wanted to use was too expensive to risk it so I did it this way and am very impressed with how it came out. Did the entire underside of my '53 and oh, did the trunk floor too.
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