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rust removal using phosphoric acid- HELP

Discussion in 'The Hokey Ass Message Board' started by savannahDan, Oct 24, 2011.

  1. savannahDan
    Joined: Nov 26, 2008
    Posts: 131


    I'm working on a 39 ford tudor that has some pretty decent surface rust on the roof and hood. I first sanded everything down with 40 grit on a DA sander. After that I wire wheeled it real good to clean out the pits. After that, I applied some Jasco Prep and Prime which is phosporic acid. I can see that the acid has done 3 things: dissolved some rust, turned some rust black, and left some chalky white deposits. Of course, I'd like for all the rust to be gone, but I'm okay with it turning black, which is iron phosphate from what I've read and it's okay to paint over. However, I can't get the white scaly stuff off, I've tried more 40 grit on the DA, more wire wheeling, stripping discs on angle grinder, nothing is removing it. Is it okay to paint over? If not, how can I get it off? I have a sandblaster as a last resort, but I'm really apprehensive about using it on a big open panel like the roof.
  2. chrisntx
    Joined: Jan 20, 2006
    Posts: 1,791

    from Texas .

    The white residue can be removed by washing with 1/2 phosphoric acid and 1/2 water mixed.
    Apply the mix, wipe off with a dry cloth, use air pressure to force it out of the rust pits, keep wiping
  3. Loppy
    Joined: Oct 22, 2007
    Posts: 422


    Poke at the black. Those are areas that the phosporic acid has puddled-up, or has reacted with foreign matter like dirt, oil, grease, paint, body filler, etc... If it's soft, remove it down to the steel and re-apply the phosporic acid. If is hard, the phosporic acid has done its job. If its hard and cracks off, sand off the parts that want to leave. The white scaly stuff is fine, it's areas that just don't have that much rust. Just knock it down (sand) with 320 or red Scotch Brite then blow off real good. After that do the normal surface prep and your good-to-go to primer. I usually spritz and wipe with a good wax and grease remover or "post sanding" cleaner, then tack while I blow it off.

    If it makes you feel better, something is making it stick. So odds are it isn't coming up. Phosporic acid's have been used for years with great results. Go for it.

    This is what Jasco states on the web site:

    Directions for use
    EXTERIOR/INTERIOR PREP: Wire brush or sand scaling paint, dirt, loose or heavily rusted area. Use paint thinner for oil and grease removal.

    APPLICATION: Wear chemical-resistant gloves and chemical splash goggles. Apply Prep & Primer directly to metal by brush, roller, mop, or low pressure spray. Heavy rust may need two applications.

    FINISH - RUSTED AND OLD METAL: Wait overnight after application of Prep and Primer, dust off any loose powder, apply undercoat primer and paint. GALVANIZED: Paint can be applied when Prep and Primer is thoroughly rinsed within 30–60 minutes, and dried.

    CLEANUP: Clean application equipment with water.

    COVERAGE: Approximately 150 sq.ft. per quart.

    HELPFUL TIPS: Test small area for compatibility when using epoxy or latex paint. Follow paint manufacturers instructions for undercoats and top coats. Protect plants, concrete, masonry and paint from overspray and flush immediately with water if contact is made. Do not use an airless sprayer or dilute this product. Do not use when temperature is below 36º F.
  4. Loppy
    Joined: Oct 22, 2007
    Posts: 422


    ...and use eye protection!
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  5. savannahDan
    Joined: Nov 26, 2008
    Posts: 131


    thanks for the feedback so far.

    For what it's worth it seems the white stuff is on top of the black stuff that is down in the pits.

    That's a good point you made about if it's so hard to remove, it should be okay to paint over it. Still seems weird to paint over a porous white scale though.....

    Chrisntx, I've used more of the Jasco in an effort to soften up the white stuff and try to scrub it off, but that hasn't worked either. I've been using various color scotch brite pads, steel wool, and a wire cup brush on a drill on slow speed, all while the acid is still a little wet, but no results. On flat smooth areas, it's cleaning up real good, but nothing seems to be getting to it in the pits, or it's just really bonded to the iron phosphate.
  6. 85-percent
    Joined: Apr 5, 2005
    Posts: 323


    I've always had wonderful luck with etching wash at the auto body supply. I use it for rust removal as a first step. this drys and reveals any more rust areas that need grinding, sanding, blasting, wire wheel, before final.

    The final application immediately preceding the primer coat is done with the following steps - dilute concentrate with water 1 part prep in five parts water. (approximately, i usually mix it by instinct.)

    wet an area about 1 to 2 square feet at a time. wet it down with the wet cloth until you change color to the metal. Now, while all is wet, use as many dry towels as needed (I use bath towels gone bad scissored up into 6" squares.) and wipe clean and wipe clean and wipe and wipe until the panel is as dry as you can make it. I then wait an hour or two before shooting. The final metal color has shades of purple and red and blue in it, but is mostly clean grey metal color. If I get one or two streaks that are white enough to make me wary, I redo this procedure and it usually removes the whitish streaks, which I assume is too much left over acid that didn't get removed correctly?

    Now, for HEAVY RUST, I use muriatic acid, which is not the nicest stuff environmentally. It will eat rust like corpses in acid vats in 50's horror movies. I think it also eats the parent metal, but not as quickly as the rust. a couple of rinses and some time, and then do the metal prep as per above.

    ONLY USE THIS STUFF OUTDOORS - it will eat up all bare metal in your garage from the vapors alone! I would not use it if it wont get off all the rust. muriatic will stay in any rust left and wreck the paint job. also need rubber gloves for this stuff.

    only do small areas. dont quit halfway - the wait will wreck your part. I would advise letting the part sit for a couple of weeks if any lap joints or other places where the muriatic didnt get removed are possible. only use it if the part surface is continuous with no overlapping and spot welding! it may take metal prep redo's on all seams to get the muriatic totally gone.

    I never use hardware rust removers for final wash - i always go with metalprep for bare metal. i do use the gels and other home depot brews for initial derusting. also, let it sit and repeat repeatedly until rust is driven from the land.

    respect all acids and dont unintentionally do anything to mess up your health or your project or other stuff nearby!

    I've always been fascinated with letting the chemicals do the work instead of all that grinding and wheeling and blasting to remove rust!

  7. enfieldjoe
    Joined: Jun 5, 2009
    Posts: 839

    from Eustis, FL

  8. TekHousE
    Joined: Apr 30, 2014
    Posts: 2


    That white build up will most likely not allow paint to adhere! Phosphoric acid attacks rust and converts it not removes it. It ALSO will attack metal very slowly. So it is best to remove it after reaction has taken place. DO not use it on metal parts that are stabilizing or otherwise strength critical! Such as leaf springs. It works best on sheet metal panels etc.
  9. I love phosphoric acid for rust removal. Unlike most other acids, it doesn't promote rust after it removes it. I've treated parts and had them sit for months in the shop without re-rusting. Check the tech sheets for your primer though. Some epoxy primers will not adhere to any phos acid on the surface. In that case you may need to rinse and neutralize the surface before priming.

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