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More on prep for painting using phosphoric acid such as Ospho with pic

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  • More on prep for painting using phosphoric acid such as Ospho with pic

    I am forever promoting using this prep for freshly blasted metal or otherwise cleaned.
    This is a hood from the M5 I am working on. Like all the other sheet metal, it was water blasted a year ago. The water blasters put in a special chemical to stop it from flash rusting, but that stuff only lasts a day or two.

    Few people are prepared to prime their sheet metal immediately, esp since it will get in the way of any metal repair you have to do, which is usually considerable.

    You don't need to. We buy Ospho etc in gallon bottles and load into a garden sprayer, and spray down the tin as the blaster is done with each piece. It matters NOT if the resultant appearance of the surface is uniform, just spray it on there, leave in sun to let it dry. DO NOT wash it off, that is a huge mistake.

    Now find a safe indoors place to store your parts until you are ready to attack each one properly. The parts WILL NOT rust while in storage if have followed these instructions.

    This hood was blasted and phosphated a year ago, and I am just now getting to metal repair.
    You can see how the surface color is irregular, NO MATTER. That battleship gray color is just what you want. It tends to be a bit chalky, NO MATTER.

    The next steps in order to prime:
    1. Go over the entire panel with a scotch brite pad.
    2. Blow off with compressed air, be sure to use a filter ball ahead of your airtip to trap any oil moisture from the compressor.
    3. lightly sand with 400 dry paper if the surface is not smooth enough for your liking. Again, blow off when done. You are NOT trying to get back to bare steel, in fact this is what you don't want.
    The chalkiness will disappear using the scotch brite.
    4.Your metal is now ready to accept primer of your choice. This is the best coating available to accept paint. The highly reactive steel has been 'passivated' (google it) by the phosphoric acid to become a coating of iron phosphate, which is very chemically stable. This method has been around at least since the '40s. I have a handbook of chemistry from the '40s and it describes it there

    We do use nitrile or vinyl gloves when handling the acid, but really now, phosphoric acid is the main ingredient of coca-cola, so just how bad can it be.

    I have a spare Model A frame in the corner that received this treatment 40 yrs ago, looks like I did it yesterday. I learned this method from a high end restorer back east where if you failed to follow this method, you paint job would start to peel and blister in 5 years NO MATTER WHAT ELSE YOU DID because of the excess humidity

    Cast iron such as cylinder heads can be treated the exact same way; anything ferrous.

    Last edited by tbirdtbird; 02-19-2018, 11:02 PM.

  • #2
    tbird, so you are saying that if I use oven cleaner on a block and wash it off with the Ospho it will seal the block even if I run the engine and paint it in say 3 days?
    You wana look waaay far up da road and plan yer route because the brakes are far more of a suggestion than a command!


    • #3
      Ummm, I would wash the oven cleaner off with water as per usual. Then I would slather it with Ospho. You would want an excess, since oven cleaner is lye and there will be a slight residue that will neutralize some of the Ospho. Then let the Ospho dry thoroughly, either in the sun, heated garage, or we have even played a MAPP torch over the metal very lightly to acclerate drying.
      In the case of cast, I don't think a scotch brite would clean off the excess Ospho very well, we usually would use a small, new wire brush, then hit it with the air tip.

      Of course for the general readers out there you would have to be careful of the oven cleaner; I know that you, the Wiz, know this

      Ospho is not a seal, it will however get into all the tiny pores of the metal that blasting, oven cleaner, and paint can't get to, where usually tiny specs of rust lurk, just waiting to break thru your paint job in 3 yrs.

      Then you would apply the engine paint of your choice, of course with no primer on blocks or heads

      But yes, you could use the Ospho and paint the engine later if that is your question. BUT you would want to be sure there was no contamination of the Ospho surface from oil mist, coolant leakage, etc, anything that would come from a running engine that would give you fish eye on your paint. I am not sure such a clean running engine exists. The oil mist coming from the filler cap alone would stop me from running the motor before I painted it
      Last edited by tbirdtbird; 02-19-2018, 11:38 PM.


      • #4
        Dave, I always had a gallon on board. Throw in a handful of rusty nuts and bolts and next morning they looked good as new. Stainless steel only means it stains less.


        • #5
          I do it that way and have for years. One thing I wipe the Ospho on instead of spraying. That stuff goes a long way and more is not better.


          • tbirdtbird
            tbirdtbird commented
            Editing a comment
            maybe. I knew a guy who wiped it on freshly blasted panels, he must have gone too light with it; it didn't get all the pores, and he had some tiny rust spots popping thru in 3 yrs.

            Blasting and wire wheeling panels and heads and such does not guarantee you got into all the tiny microscopic pits where the residual rust is hiding.

            Having said that you must have a good technique because it does seem to work for you

        • #6
          I am going to Ace to get a gallon as we speak. I have heard of this but never tried it yet. Blasting all these parts now and will start using it. It is good for body panels also, correct?


          • tbirdtbird
            tbirdtbird commented
            Editing a comment
            Do a test piece.
            We also load up a plant mister with it to spray smaller panels and small parts

        • #7
          Absolutely, anything ferrous


          • #8
            Do you spray it on full strength or does it have to be cut with water like Metal Prep?


            • #9
              I remember the days of DuPont Metal Prep, it is the same ingredient, of course.
              The Ospho must be a little weaker because when we diluted it we did not like the way it looked, did not seem as effective. The metal would take on a very slight orange haze in places which told us the passivation was not complete

              For any members in FL, there is a brand there called Fozz that can be cut 50% with water
              Last edited by tbirdtbird; 02-20-2018, 10:43 PM.


              • #10
                Correction.....I brush Ospho on not wipe it on. TbirdTbird comment about the little holes is correct.
                Last edited by Captndan; 02-21-2018, 09:20 AM. Reason: spelling and still isn’t right


                • tbirdtbird
                  tbirdtbird commented
                  Editing a comment
                  Captain, that would work just fine IMO!!!!

              • #11
                Question on this older tread....I have a 1929 Leatherback 60B and I removed some metal skin parts(header metal cover, windshield/door post piece, door header metal) that all have surface rust on the back side(front showing surface is good).
                how do you recommend prepping and painting the backsides before I reinstall? I will tape off the finished outside painted surface. Just wondering if the Ospho would bleed/or penetrate through the backside of the skin metal and effect finished painted side. ???
                Last edited by Ken Harvey; 02-03-2021, 05:35 PM.


                • #12
                  A number of years ago I talked with the owner of SPI for a while. He was quite forthcoming with technical information. One point stuck out in my head. He stated that he would not honor warranty on Epoxy primer over any acid coatings. He told me that any leftover acid on the surface would prevent the epoxy from curing. Epoxy uses a base to cause the chemical reaction for curing.

                  Today I was on the Vintage Mustang Forum and found this:

                  I just went through this with Ospho as I wanted to use SPI epoxy after applying it. I spoke to one of their technical support people about it and this is what they told me.

                  To use ospho, follow the directions exactly to apply the ospho, and leave it on for the recommended time. When you want to remove it, apply a thin layer of ospho over the area you already applied it to, let sit for 5-10 minutes, then wipe off with soapy water (I used dawn). Dry with compressed air. Applying the new coat of ospho “reactivates it” and allows you to remove it.

                  So something to consider when you are using acid products before epoxy paints.


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