Clear Coat on Anodized Aluminum?
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    Default Clear Coat on Anodized Aluminum?

    Hi All,

    This forum is my go-to place for answers, so thanks in advance to all of the members here. This is probably a very newbie questions but here goes:

    I have a part that I machined and have anodized, but it is slightly more matte finish than I want. It is a mixture control knob for a flight simulator:

    img_1622.jpg

    I wasn't able to buff or sand it perfectly after blasting due to the geometry and being worried about rolling over the corners. As such, it is perfect except slightly less shiny than I want.

    Can I put some sort of Lacquer or Polyurethane coat on top of anodized aluminum? I would just want a little more gloss, but something that can stand up to being handled a lot.

    Thanks!
    Charlie

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    Not the answer you want but if it was me the last thing I'd want was a shiny knob on something I'd be using outdoors on a sunny day. Is there a special reason you want it shiny and how shiny is "shiny"?

    BTW it would be a good idea to add your location as a member.

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    Quote Originally Posted by Gordon B. Clarke View Post
    the last thing I'd want was a shiny knob on something I'd be using outdoors on a sunny day.
    The simulator is basically a giant computer game. It is played indoors only. There is a panel built to match an airplane, and this is one of the knobs on it.

    Quote Originally Posted by Gordon B. Clarke View Post
    Is there a special reason you want it shiny and how shiny is "shiny"?
    Great question - the reason I want it shiny is that I have prepared two other sets of knobs that I was able to buff a nice polish onto before anodizing. I was hoping to make the finishes more similar, even if they don't match perfectly.

    Quote Originally Posted by Gordon B. Clarke View Post
    BTW it would be a good idea to add your location as a member.
    Adding it now!

    Thanks,
    Charlie

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    Any coating that you put on it will be prone to wearing off. That being said, modern polyurethane finishes, even the water based ones, are very durable and should last a long time. I would use several coats.

    Perhaps there are some tricks when anodizing. I am not familiar with that process.

    Of course, you could dull the other knobs to match this one.

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    Just buff the finish after machining, then anodize the parts. I've seen a product used as gravel guard clear coat. Used on cars and truck fenders to keep from chipping paint and it's extremely tuff stuff , very hard to ware off. I think one is sold as gravel guard.

    Blasting with plastic bead or walnut shell would give a better finish , if not a small tumbler vibrator sold at HF or NT online.

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    Anodized surfaces are not normally paintable IMO.

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    Quote Originally Posted by scadvice View Post
    Anodized surfaces are not normally paintable IMO.
    Why do you say that? Many painting specs allow for anodize before paint. I would expect it to improve adhesion.

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    For the future have you considerd Electropolish before anodize?

    Electropolishing as a Pretreatment for Anodizing : Products Finishing

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    Clear powder coat it.

    If you're in the US check out Eastwood, they have powder coating kits for a good price. I have one, and a commercial unit too, and typically use the Eastwood version for little things like this. No connection to the company etc etc.

    Powder Coating Gun - Hotcoat Powder Coat Guns - Powder Coating Kit

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    I've done a lot of experimentation with giving aluminum parts that special sheen. I'm guessing what you're after is a metallic finish that still has the iridescence qualities about it that scream "Machined Part!"

    We started having to experiment with this after a primary component on one of our camera straps was regularly accused of being made of plastic by our customers. The part was quite well machined, tumbled, bead blasted and Type III black anodized. People literally wrote in saying "Gee, this thing works great, but I don't know about hanging my camera from plastic like that..."

    The finish was changed quickly...

    What we do now:

    1- We focus on really high quality finishes straight out of the machine. It takes only a few more seconds on a modern 3+2 or 5 axis mill to do a really nice finishing pass and get everything looking pretty fantastic.

    2- Every edge is on-machine de-burred. Fillets and chamfers are primary design features that get integrated into the design early. We design every single feature around providing tool access to hit every single edge, and I've gone as far as calling out Harvey or Lakeshore Carbide part numbers on prints, and running designs through CAM to prove out paths, simulate access, and get some idea of where designs are just going to take forever to get smooth. The end result is that parts for us come off the machine ready to use. No polishing, manual de-burring or other surface finishing procedures are ever applied.

    3-Be OK with letting some tool path shine through. Polishing will break edges and often do more harm than good. We're to the point of writing our own g-code on some finishing passes, buying our own cutters/holders and giving up some machine time efficiency just so we can make tool path an integral part of the design aesthetic. (Yea, I am a pain in the ass customer sometimes).

    4- We've developed a number of anodizing recipes that get us the exact finishes we want. I'll give up the trade secret though; if you want the freshly machined look while still having anodizing protection and color, our shop does an R8 bright dip for about 40 seconds, and uses a Type III tank (lower temp, higher current) to build a layer of anodize on the part that is **just** past the iridescent point (this amount will change for every part). Essentially, we're stopping the anodize process the second that a homogenous, uniform layer has built up.

    This actually holds up quite well because of step 2- every edge is chamfered or filleted so we absolutely minimize the sharps that have a tendency to anodize thin and wear out rapidly.

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    I'm overwhelmed by the great replies, thank you so much everyone. To update, I used a test piece of aluminum and tried a bartop finish I got at home depot (thinking that it would be a very durable clear finish). It didn't really work at all. But now I have all of your replies to work from!

    Any coating that you put on it will be prone to wearing off.
    That is exactly my fear. I know this will be handled a lot so I am looking for as tough a finish as I can reasonably expect. Will read about the finishes you mentioned. Dulling the finishes of the other knobs is not exactly ideal, but yes it is another option! Was hoping to bring the straggler up to muster.

    Many painting specs allow for anodize before paint. I would expect it to improve adhesion.
    This is my assumption as well, the surface is nice and rough and clean.

    have you considerd Electropolish before anodize?
    Honestly, didn't even know about it. I am going to read into it now.

    Clear powder coat it.
    Never occurred to me! I have a powder coating kit my wife gave me for xmas. I just didn't think clear would be an option with it!

    gkoenig - thank you so much for your detailed reply. It is really helpful to see exactly what works for others. Your recommendation of bright dipping seems to be the same thinking that Atomkinder was going towards. Also, my main problem was trying to use blast media (Armex, Walnut, Alu Oxide) to remove the machine marks. I knew I would have to do this to get rid of the marks, but I didn't want to lose the features of the part. I ended up only blasting the "Sides" of the knob which made it much more dull than the peaks. I tried tumbling in corn cob media and brasso, hand buffing, but got very concerned with rounding over features.

    I think in the future the correct answer (as always) is having the part prepared properly before you anodize it. After anodizing the ship has kind of sailed. This is the first part I've ever anodized, so I'll consider that a very important lesson.

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    In the late 1960s I made a new set of panel knobs for a Cessna 195. The plated steel ones were getting ratty, so I made new from 6061 and polished them on the lathe, getting an almost polished finish with very fine lines, a sort of misty effect, to keep it from looking like a dime store piece. I got them anodized, I don't know what class, but it was clear, and when I sold the plane a couple of years ago, they still looked good.

    Bill

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    Quote Originally Posted by 9100 View Post
    In the late 1960s I made a new set of panel knobs for a Cessna 195. The plated steel ones were getting ratty, so I made new from 6061 and polished them on the lathe, getting an almost polished finish with very fine lines, a sort of misty effect, to keep it from looking like a dime store piece. I got them anodized, I don't know what class, but it was clear, and when I sold the plane a couple of years ago, they still looked good.
    The pilot I am making these for specified that they be made to fit the simulator, but he has asked a couple of times if they could fit in the plane too now that he has seen them. I think he is worried about FAA part requirements, but regardless, I don't mind making "simulator" knobs for him!

    I am a student pilot right now, looking forward to having something to make knobs for one day!

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    Clear powder coat is tough and will last many years. The clear coat lasts longer if not exposed to UV (it can become yellowish if exposed often to the sun). I made many aluminum parts that were hand polished in a buffer to get close to a mirror-like finish and then powder coated clear (parts were not anodized). They looked great, although buffing them by hand sometimes didn't achieve an even finish. I don't think the anodizing will negatively affect the adherence of the powdercoat, in fact it may even help.

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    Quote Originally Posted by astromach View Post
    Clear powder coat is tough and will last many years.
    I am going to have to investigate clear powder coating - I for some reason didn't even consider powder coating. Maybe I just assumed it wouldn't be as high of a finish? Regardless, will investigate.

    UPDATE:

    I tried the bartop epoxy coating, and it left a very bad and lumpy finish on a test part. I decided I needed to spray applicate somehow or i'd risk ruining the finish due to a heavy brush.

    I got some rustoleum clear gloss enamel spray and it did very nicely on a test part. Just a very thin spray. I am planning on using this just to bring up the shine a little bit, and telling the customer that if he finds the finish dulls I can reanodize it for him at no cost. That way he gets it on time, and if he notices it dulling I will try maybe chemically brightening or prepping the part before a second round of anodizing.

    I am still looking at this thread to see what other people recommend, and I really appreciate your help. This forum is just awesome.

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    Yellowing of clears is, so far as I've seen, not applicable these days. The early powders weren't UV stabilised, hence PC gained a reputation for yellowing and fading. That was a generation ago, and the modern powders perform similarly to conventional solvent based paints these days. Only much better. A lot of products that we assume to be bare metal are actually PCed to preserve the finish, prevent oxidisation, and provide a more durable finish for soft metals. Wheels on cars is a good example.

    Here are your clears. Powder Coating Powder - Hotcoat Powders From Eastwood however I prefer Powder 365, Powder365, Customer Service Like No Other drop Cindy a line up there. She's super efficient, the customer service is great, and, best of all, she's an Aussie!

    Personally however I wouldn't anodise and then PC it. If I were going to PC the part I'd go straight to a powder coat of the correct colour in a high gloss finish.

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    Quote Originally Posted by Pete F View Post
    Here are your clears. Powder Coating Powder - Hotcoat Powders From Eastwood however I prefer Powder 365, Powder365, Customer Service Like No Other drop Cindy a line up there.
    Thank you so much for the product links. Helps a ton. I agree in the future that if you want to clear coat you should just do that and not anodize first. Twice the work for no reason.

    In this case, that is a viable solution, since the anodize is already done and it is simply fixing the finish... but my big lesson here is to make sure I get that bright finish before I anodize, and be more careful next time before throwing the parts in the blasting cabinet!

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    Quote Originally Posted by Atomkinder View Post
    For the future have you considerd Electropolish before anodize?

    Electropolishing as a Pretreatment for Anodizing : Products Finishing
    Electropolishing is a good way to go. You can get close to doubling your surface finish, just expect to lose a few tenths of material.

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    Quote Originally Posted by robert123 View Post
    Why do you say that? Many painting specs allow for anodize before paint. I would expect it to improve adhesion.
    It only improves if you don't have the type II anodize sealed. Aluminum being a soft substrate needs a hard coat for the finish to cling to. If I was going to have something clear finished then I would have it clear chem filmed and not anodized.

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    That's a pretty fancy knob for a flight simulator. What kind of simulator is it?


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