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Thread: Coloring aluminum castings

  1. #1
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    Coloring aluminum castings

    I'm casting 1/4 size "cast iron pigs" of aluminum for sale at our open house and festivals (www.haycreek.org) Can you suggest how to color them to look like cast iron? One idea was to use a 10% muriatic acid solution as a dye. I don't want to use a pigmented paint.

  2. #2
    You can use drain cleaner to darken/etch aluminum. I have been using Draino from Home Depot. If left in the solution for about two hours, the surface turns black. The black will rinse off with soap and water, leaving a darker surface than prior to acid dipping. You may want to stop there, or you can lightly apply black enamel paint which seems to adhere to the metal much better after the acid bath. Thoroughly clean the piece with soapy hot water first The paint layer must be thin or it tends to flake off. I brush it on. Leaving it in acid longer achieves a darker surface after rinsing, and you should time your acid baths so you have an idea of how long you want to leave the pieces in. If you leave the metal in acid for extended time (5-6 hours) it will turn black, but after rinsing, it will appear chalky gray. This too can be painted IMG_1109.jpgIMG_0300.jpgIMG_1112.jpg

    Pictured are some pieces I left in acid for a few hours (multiple black skulls), then scrubbed the black stuff off (same skulls but lighter), and a piece I left in longer, which was scrubbed but remains black. There will always be a layer of black gunk that has to be scrubbed off, but under that the metal will be darker depending on how long it's left in the acid.
    DISCLAIMER: The black surface may not be completely uniform, and you may even notice white dots appearing on the really dark pieces. I don't know why this is, but I presume it's a result of corrosion.
    Be careful with the acid, wear rubber gloves, googles, and do not breath in vapors. I do this outdoors to avoid vapor buildup, but even then you have to be careful while working directly over acid tubs. Use plastic tubs, and I usually guesstimate how much draining to use. I use small containers so I'd say I use about 1/4 cup of draino to a half gallon of water.
    Hope this helps.
    Last edited by jerseydevil; 04-06-2017 at 03:43 PM.

  3. #3
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    Are you sure you are using acid? The Drano I was always familiar with when I lived in the US was not an acid but a strong alkaline, sodium hydroxide I believe it was. The chemical opposite of an acid.

    But they may have changed their product.

    Richard
    When I die, Heaven can wait—I want to go to McMaster-Carr.

  4. #4
    Related to this discussion, what do you guys use to pretreat the aluminum for a good paint job to get good adhesion. I have always used alumaprep (acid) and then Alodine , just wondered if there is a less expensive, readily available, maybe less toxic, homebrew that works?

  5. #5
    Quote Originally Posted by Rasper View Post
    Are you sure you are using acid? The Drano I was always familiar with when I lived in the US was not an acid but a strong alkaline, sodium hydroxide I believe it was. The chemical opposite of an acid.

    But they may have changed their product.

    Richard
    You are correct, Draino is sodium hydroxide. I guess I always thought of it as acid because it eats away at stuff. Thanks for pointing that out. All of the mentioned safety precautions should still be followed.

    Planebuilder, I have had carrying success with washing with soap and hot water, then wiping down with acetone, then using duracoatp adhesion promoting primer, then duracoat finish coat. I've also tried rustolium etching primer. I've also. Success with no primer, and just enamele paint.

  6. #6
    Senior Member cactusdreams's Avatar
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    If you're just going for the black iron finish like a Lodge skillet you might try the polymerized oil method. Don't know if the aluminum would melt but worth a try on some scrap. This was on cast iron though.

    http://vid1379.photobucket.com/album...psd2tj7gke.mp4
    People who say it cannot be done should not interrupt those who are doing it.

  7. #7
    It works on Aluminum. Had a transmission case that cracked. Tried to weld it... Let just say it was oil impregnated and as we heated up to weld the oil would run out of the metal and burn. You can clean it off with a wire brush and or solvents ... so it doesn't stick as well as to cast Iron. But it will work. Doesn't weld for poo... but pretty black/brown all over.

  8. #8
    A black Sharpie pen works pretty well on aluminum if you let it dry for a few hours - but of course, it does contain pigment.

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