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Thread: Phosphoric acid metal prep

  1. #1
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    Default Phosphoric acid metal prep

    Good evening

    I am prepping some bare metal fenders for painting. I plan to clean with mineral spirits and use a self etching primer for the top followed by a good topcoat.

    On the underside, I would like to use a zinc rich paint (aka cold galvanize). Iím on the fence about wether that stuff is snake oil but am leaning towards using it anyway. Therefore, I was looking at phosphoric acid to etch the surface prior to painting.

    My question is whether I need to neutralize the acid residue with baking soda after washing the surface or can i just paint after it is rinsed and dry?

    Thanks

    Tom

  2. #2
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    Default Re: Phosphoric acid metal prep

    I would just use self-etching primer on the clean bare metal, then recoat (without sanding) within the specified time window.

  3. #3
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    Default Re: Phosphoric acid metal prep

    Phosphoric acid (U.S. brand name 'Ospho') can be used without any neutralizer. It converts rust to a hard, inert iron phosphate. Wire brush any loose bits off first. Heavy rust might require two coats. When all the rust turns black, you're done. Unless you want to use bondo or similar to fill any remaining pitting. It sometimes 'blushes' a white powder, which is simply wiped/brushed off before painting. If there's no rust, there's no need for it.
    David G
    Harbor Woodworks
    https://www.facebook.com/HarborWoodworks/

    "It was a Sunday morning and Goddard gave thanks that there were still places where one could worship in temples not made by human hands." -- L. F. Herreshoff (The Compleat Cruiser)

  4. #4
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    Default Re: Phosphoric acid metal prep

    Agree with DG about oshpo, would add that regarding the zinc sprays (cold galvanize and the like) I've had very good results in situations where the softness of the coating is not a problem-it is pretty soft. Underside of fenders might be a good place, if you dont drive on a lot of gravel.

  5. #5
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    Default Re: Phosphoric acid metal prep

    Agreed. It seems like a pretty abused area. I was thinking about the zinc as a first layer, followed by a latex topcoat (as required if topcoated), then finished with bed liner.

    Thanks for the replies and good info.

    Regards

    Tom

  6. #6
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    Default Re: Phosphoric acid metal prep

    Quote Originally Posted by bluedog225 View Post
    Agreed. It seems like a pretty abused area. I was thinking about the zinc as a first layer, followed by a latex topcoat (as required if topcoated), then finished with bed liner.

    Thanks for the replies and good info.

    Regards

    Tom
    It strikes me that you are mixing and matching components in a way that is more likely to fail than to succeed. Take your issue to a local pro paint supplier, and solicit their advice.
    David G
    Harbor Woodworks
    https://www.facebook.com/HarborWoodworks/

    "It was a Sunday morning and Goddard gave thanks that there were still places where one could worship in temples not made by human hands." -- L. F. Herreshoff (The Compleat Cruiser)

  7. #7
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    Default Re: Phosphoric acid metal prep

    You may be right. I was surprised by the latex topcoat requirement.

  8. #8
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    Default Re: Phosphoric acid metal prep

    I agree with above about ospho, though it sometimes bleeds from under the paint, it needs to dry well. I also agree about getting pro advice first. But I would also say that I think the most durable coating for the underside would be spray-on bedliner, if not too heavy, that stuff seems tough as snot. But ask the pros.
    When you can take the pebble from my hand, it will be time for you to leave.

  9. #9
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    Default Re: Phosphoric acid metal prep

    Just a side note, like a lot of folks I spray Ospho onto any rust streaks on my white hull and watch them magically disappear. I was a little concerned about the environmental affects of it in the water so I did a little basic research into its chemistry. I was amazed to discover how similar Ospho is to Coca-Cola. Happy drinking.

  10. #10
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    Default Re: Phosphoric acid metal prep

    Curious- do you undercoat your care when new? I’ve brushed off surface rust as needed on undercarriage, primed after,
    There's a lot of things they didn't tell me when I signed on with this outfit....

  11. #11
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    Default Re: Phosphoric acid metal prep

    I used to use a product called Por15 on frames, inside fenders etc.
    Investigate it before committing to galv. Finish is almost like powder coat. Rock hard.
    "Many a time freedom has been rolled back - and always for the same sorry reason: fear." - Molly Ivins

  12. #12
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    Default Re: Phosphoric acid metal prep

    Good tip.
    There's a lot of things they didn't tell me when I signed on with this outfit....

  13. #13
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    Default Re: Phosphoric acid metal prep

    Quote Originally Posted by bluedog225 View Post
    I’m on the fence about wether that stuff is snake oil
    it is

    sign me, maintainer of about 193 miles of chainlink fence
    Simpler is better, except when complicated looks really cool.

  14. #14
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    Default Re: Phosphoric acid metal prep

    Quote Originally Posted by Paul Pless View Post
    it is

    sign me, maintainer of about 193 miles of chainlink fence
    Snake oil? Hardly. If used as directed, for the purposes intended. It works well.

    If you use it as a finish, and not just a rust-consolidating and halting bit of prep, it won't work. Not intended as a finish.
    David G
    Harbor Woodworks
    https://www.facebook.com/HarborWoodworks/

    "It was a Sunday morning and Goddard gave thanks that there were still places where one could worship in temples not made by human hands." -- L. F. Herreshoff (The Compleat Cruiser)

  15. #15
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    Default Re: Phosphoric acid metal prep

    We build a lot of "soldier beam" retaining walls and the call out for the finish is very strict if it is a state job. All steel must be sandblasted and immediately coated with a zinc rich self etching primer, then an epoxy top coat. Supposed to be good for 100 years buried under ground. The two part coatings are expensive.
    The county engineers will sign off on brush coated single part zinc paint. 75% zinc by volume I have used it on my boat trailer axles, still like new 15 years later.

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