DevPrep 88 alkaline de-greasing before painting

Discussion in 'Metal Boat Building' started by Steelboat, Mar 22, 2023.

  1. Steelboat
    Joined: Feb 2022
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    Location: Seattle

    Steelboat Junior Member

    I have read through comments on use of acids like phosphoric on interior rust, and agree that is is best avoided.

    I am prepping some dirty oily rusted areas, and considering an alkaline cleaner instead.

    I can't get Devprep88 where I am working, so considering making a solution of water based detergent and weak trisodium phosphate. Lots of rinsing after of course.

    Any experience with alkaline prep wash?
     
  2. gonzo
    Joined: Aug 2002
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    gonzo Senior Member

    Phosphoric acid is a time tested treatment for steel. It works fine. You can use any strong detergent cleaner for oily areas. Ideally, use a hot water pressure washer with a detergent mixer. It adds the detergent to the water at the suction side. If they are really built up with oil and grease, you can first clean them with kerosene or diesel fuel and a wire brush.
     
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  3. Steelboat
    Joined: Feb 2022
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    Location: Seattle

    Steelboat Junior Member

    Thanks Gonzo. Big controversy on using phosphoric acid on boats. The paint technical support people all told me to never use an acid, especially around pitting, as likely acid will remain there.

    I also read that an alkaline environment discourages rust, so my thinking was that an alkaline de-greaser would be good.

    Naturally the best is blast to white metal, followed by proper epoxy primer. Many paint companies claim "surface tolerant" primers that can go on over tight rust. The conversion coatings people all say good results over tight rust. Doubtful in my experience.

    So the question is, if you just can't blast, what is the second best choice? I plan to de-grease, grind, needle gun and wire wheel, followed next by one of these:

    1) Devoe 167 penetrating epoxy primer (very thin)
    2) Zinc-rich moisture cure urethane primer
    3) Phos acid conversion treatment
    4) Traditional epoxy primer
    5) POR-15 or similar conversion coating
    6) Tannic acid treatment

    All seem to have advocates.

    After that I plan to build up the coating system with epoxy primers, likely coal tar as it is in the bilge areas.
     
  4. comfisherman
    Joined: Apr 2009
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    Location: Alaska

    comfisherman Senior Member

    My previous life owning a steel boat we tried almost every combination of otc preps. Sanders, needle guns, acid wash, grinding, converters.... you name it. After about ten years we bought a blast setup and ended up with the bummer conclusion that nothing even came close to long term adhesion. It sucked doing inside tanks, lazarettes and fish holds but the product held up long term.

    Began to be a game of masking and air management to blast everything that needed paint. Was a miserable job, but nothing came close to keeping paint on steel.
     
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  5. Steelboat
    Joined: Feb 2022
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    Location: Seattle

    Steelboat Junior Member

    Amen to that. I bought blasting gear too, and even did the inside of a lazerette. It was blast, stop, let the air evacuate then look at work, repeat 100 times.

    There is just some cases where even wet blasting is too much, like around bilge frames of fully built sailboat, literally tons of woodwork all around. In this situation we need a second best option.
     
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