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Applying undercoating over surface rust?

Discussion in 'The Hokey Ass Message Board' started by Stevie Nash, Mar 8, 2012.

  1. Stevie Nash
    Joined: Oct 24, 2007
    Posts: 2,999

    Stevie Nash

    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...
  2. Bigchuck
    Joined: Oct 23, 2007
    Posts: 1,159

    from Austin, TX

    Kill the rust first. OSPHO, or something similar.
  3. CutawayAl
    Joined: Aug 3, 2009
    Posts: 2,144

    from MI

    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).
    Last edited: Mar 8, 2012
  4. hillbilly4008
    Joined: Feb 13, 2009
    Posts: 2,923

    from Rome NY

    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...

  5. unkledaddy
    Joined: Jul 21, 2006
    Posts: 2,865


    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.
  6. rustednutz
    Joined: Nov 20, 2010
    Posts: 1,580

    from tulsa, ok

    Undercoating no likey rust, you've got to remove or neutralize first.
  7. Crystal Blue
    Joined: Nov 18, 2008
    Posts: 609

    Crystal Blue

    Why would you want to put undercoating in the trunk ?

    Everything will get fup duck ;)
  8. Bigcheese327
    Joined: Sep 16, 2001
    Posts: 6,692


    What they said. Covering rust doesn't do anything except allow you to forget about it until it works its way somewhere else.

    Joined: Jul 16, 2006
    Posts: 2,571


    Dont put undercoating in the trunk. Paint, or a roll on bed liner product would be much better.
  10. Stevie Nash
    Joined: Oct 24, 2007
    Posts: 2,999

    Stevie Nash

    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?
  11. zzford
    Joined: May 5, 2005
    Posts: 1,823


    Go with the bedliner. Hot weather will soften up the undercoat and make a big mess on anything you keep back there.
  12. 1971BB427
    Joined: Mar 6, 2010
    Posts: 8,530

    from Oregon

    Undercoating will not ever really harden and it will stink. Don't use it in the trunk.
  13. zman
    Joined: Apr 2, 2001
    Posts: 16,716

    from Garner, NC

    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.
  14. Rich Rogers
    Joined: Apr 8, 2006
    Posts: 2,018

    Rich Rogers

    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
  15. Rusty O'Toole
    Joined: Sep 17, 2006
    Posts: 9,625

    Rusty O'Toole

    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.
  16. CutawayAl
    Joined: Aug 3, 2009
    Posts: 2,144

    from MI

    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.
  17. J53
    Joined: Aug 8, 2005
    Posts: 238

    from WI

    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.
  18. get rid of the rust first......

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