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painting freshly bead blasted aluminum casting?

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  • painting freshly bead blasted aluminum casting?

    I'm making a plate roll like VPT did. My rolls will be 24" long. I intend to power them. My buddy gave me an old worm/pinion right angle gearbox for 56C motors. It's beefy - the output shaft is 1.25". As a subproject of building the plate roll I'm rebuilding this gearbox. I got it all apart and got the shafts derusted and polished. There is some pitting but I believe with new seals it will still work.

    I took the main casting and four lesser castings over to lakeside53's yesterday and glass bead blasted them. Of course, they look incomparably better. Now I want to get some paint on them. It's been years since I painted bare aluminum. It seems to me I used some kind of self-etching primer followed by top coats. I also seem to remember I had problems with fisheyes and had to sand and recoat many times to finally get a decent finish.

    What is the best way to paint bare aluminum castings?

    metalmagpie

  • #2
    The easy way, and it may be fine. = Clean, use an appropriate etching primer, then paint it.
    The sure fire way = Sand it, clean thoroughly, immediately treat with alodine, then zinc chromate primer (or whatever the available equivalent of it is at the moment), topcoat with a couple coats of paint.
    Last edited by Joel; 06-01-2021, 07:24 PM.
    Location: North Central Texas

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    • #3
      Powder coat

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      • #4
        Wouldn't think you'd need a self-etching primer on beadblasted castings, just a normal spray can automotive primer followed by your choice of topcoat. Fisheyes are, in my experience, the result of moisture.

        You planning to use a gun or spray cans?

        -js
        There are no stupid questions. But there are lots of stupid answers. This is the internet.

        Location: SF Bay Area

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        • #5
          I find that "Sand Blasting" versus Bead blasting gives better adhesion
          Also no touching the part without gloves on .
          I second the Zinc-Chomate application suggestion and as quickly as possible after blasting

          rich
          Green Bay, WI

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          • #6
            You might try Alumiprep on the bare metal, but for some reason the recommendations say not for castings. No idea why.

            -js
            There are no stupid questions. But there are lots of stupid answers. This is the internet.

            Location: SF Bay Area

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            • #7
              PPG makes a very durable industrial primer (single-part) that works extremely well on, and is recommended for, aluminum and steel both. If you're interested I'll get you the part number.
              Southwest Utah

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              • #8
                I forgot about Alumiprep, which is a phosphoric acid based cleaner IIRC. Lye cleans the hell out of aluminum too, but it can do a good job of eating it if left in contact too long.
                The only time I have gotten fisheyes was when I didn't clean well enough and some residual oils or whatnot were missed. Silicone is the worst.
                I generally clean either with detergent and water, followed by a good rinsing, or with just alcohol - especially on smaller parts.

                The last few years I have been getting pretty lazy, and have had good luck by just sanding/blasting/whatever, cleaning and using alodine, followed by regular primer and a coat of paint.
                Location: North Central Texas

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                • #9
                  Clean it, self-etching primer, quality paint

                  Regarding the bead- vs sand-blasting adhesion issue Rich mentioned, sandblasting etches the surface and gives the finish something to grab onto, beadblasting just burnishes the surface, doesnt do much for adhesion. Companies that make firearm coatings, like cerakote, recommend against beadblasting for that exact reason

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                  • #10
                    Like Joel says, Alodine treat the surface after a thorough de-grease. Then prime /paint. Don't skip the Alodine if you want it to last... I usually apply the Alodine (diluted with water as per directions) using a Scotch-brite pad. Or blast with an abrasive as also mentioned. Glass beads polish and is a very poor coating prep.

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                    • #11
                      In the UK, we would just spray or brush Hammerite paint straight on.

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                      • #12
                        Fisheyes are, in my experience, the result of silicone.
                        we're all differently experienced.

                        I've found that once aluminum castings are blasted
                        (wheels, in my case, but close enough)
                        the etch is too aggressive. Aluminum's so reactive, it starts corroding.

                        Honestly, the Hammerite is a good idea- it goes on really thin, and you give it 3 or 4 coats.
                        DO read the recoat instructions really carefully- I have messed that up before, and it's ugly.

                        Otherwise, just about any primer for aluminum, then the topcoat that matches the primer.

                        t
                        rusting in Seattle

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                        • #13
                          Do you have a build thread for the roller? I'd like to see!

                          After blasting I just brake clean the parts, don't touch them, and right to paint.
                          Andy

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                          • #14
                            Originally posted by Jim Stewart View Post
                            Wouldn't think you'd need a self-etching primer on beadblasted castings, just a normal spray can automotive primer followed by your choice of topcoat. Fisheyes are, in my experience, the result of moisture.

                            You planning to use a gun or spray cans?

                            -js
                            Fish eyes can be caused by several things, mostly oil films from improperly prepped surfaces. Moisture in the air line is another cause.
                            Aluminum castings of poor quality with pin holes can cause what looks like fish eyes.

                            I remember when I was in my teens I was in a paint shop. There was a window repair company replacing a wind shield three bays down. The guy had a spray can of silicone and was spraying it around the edge of the window frame where the rubber gasket fits. Later that day the owner painted a car and it fish eyed
                            all over. It was from the silicone.

                            JL..................
                            Last edited by JoeLee; 06-08-2021, 10:04 PM.

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                            • #15
                              Most of the two part epoxy primers like RM Diamont EP-569 work well on aluminum. I've used that stuff for years on all my machine castings, alum. and cast iron.
                              For polyester spray fill I use Evercoat, like this stuff here. It will cover pin holes in castings. https://evercoat.com/all-products/us/?find=g2

                              JL...................

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