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Ted Mooney, P.E. RET
Pine Beach, NJ
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for Metal Finishing since 1989
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Paint over anodizing and powder coating, or remove it first?
February 15, 2021
Q. Hi there,
I got a few pairs of used Brembo-made calipers for Subaru STI/, hopefully to swap parts and get one good pair.
There are some places with deep corrosion-pitting around mounting locations, but overall they are in good conditions besides the gold paint that is flaking.
Under the paint there is a very, very thin layer black in color that looks like some corrosion prevention stuff and does not look like paint/primer/etc. It might be hard anodizing but I'm not expert to tell. Or maybe it is e-coat. It goes everywhere including into deep holes and piston bores. That layer is of course compromised where there is pitting but most of it is in good condition.
The plan is to completely rebuild them and powder coat with Prismatic. I'm going to chemically strip the paint.
The questions here are about the finish.
First question: is the paint stripping going to affect the black coating?
Second question is about the prep work. Prismatic is saying that I should sand blast the metal first to create proper structure to which PC will adhere (i.e., using aluminum oxide) but that will remove the black protecting layer. What is your experience with not doing the blasting? Is it a good idea to PC over that coating.
Third is how can I repair the protective layer after fixing the pitting and protecting other machined surfaces that will expose the bare aluminum?
I read in here that aluminum should be first chromate conversion coated, followed by an e-coat primer, followed by the powder coating.
Again applying other coats before PC is not what I found on most places describing how to apply PC.
Last one: given that most of the black layer is good, how do I fix only the problem spots? Assuming that layer is e-coat PPG has a video where they apply primer to the bare metal when repairing e-coat panels, but I'm using PC and I read that primer under is a no go. I do not have the equipment for reapplying e-coat, but even if I did (or go to a shop) I do not know if it can be done over existing e-coat. Submerging the part in the bath creates additional problems with clearance of the pistons. Now the pistons bores are already black coated.
Any help/insight is greatly appreciated.
Thank you,
- Sammamish, Washington
A. Hi Doru. Hardcoat anodizing can be light gray to light charcoal, but probably not black. I would guess that it's e-coating or auto-phoretic coating (which is usually black).
1. I would suspect that stripping the topcoat would strip this primer as well
2. If you can remove any flaking paint with steel wool or sandpaper, you should be able to powder coat over it.
3. Bare aluminum should receive chromate conversion coating and/or a primer made for aluminum. It's hard to challenge the claim that primer under powder coating is a no-go when I don't know who said so and referring to exactly what, but I disagree. Chromate conversion coating is the best pretreatment for aluminum, and e-coating / auto-phoretic coating is often done under powder coating, and probably was in this case.
In brief I would do chromate conversion coating if convenient, then prime just the bare spots per that PPG instruction you reference, then powder coating.
But all of this refers to the back side of the caliper; I wouldn't put anything on the actual braking surface.
Luck & Regards,
Ted Mooney, P.E. RET
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
February 2021
February 21, 2021
A. Hi
I'm a powder-coating caliper engineer.
The following is you should do:
Firstly:Use removal Chemical to strip powder coating (Atotech Master Remover is ok)
Secondly:Caliper pretreatment: clean - hard anodizing - chromate/seal - drying. If you like black you can dye black after anodizing. Don't sandblask, anodizing is enough.
Thirdly: clean it and paint.
Good luck!
- GZ China
⇩ Related postings, oldest first ⇩
Q. Hey new to the forum. So far all of the threads here have been a great help, however I have searched & can't figure out what to do with my motorcycle wheels and calipers. I am looking to paint the spokes on my motorcycle wheels orange and was wondering what steps I should take since they are powder coated rough cast aluminum. Also my calipers are gold anodized rough cast, and I would like to paint them blue with some high temp paint. I was just wondering what kind of steps should be taken for both projects. Either paint over both the coatings or remove them first with some easily available chemicals/etchers. Also any good masking ideas for aluminum, since I can't get regular blue masking tape to adhere well.
Sorry so many questions, but any help would be greatly appreciated, Thanks
hobbyist motorcycle enthusiast - Palmer, Massachusetts
2004
A. I'd personally go very simple with this project; no fancy chemicals needed. I'll offer that I'm no pro with any such removal processes, but I don't see any reason they'd really really be necessary.
Given that both powder coat and anodization are relatively permanent, versus paints, the coating that's already on both surfaces probably won't offer you any problems with peeling or cracking once you've got your paint applied.
What will cause you problems is any dust, dirt or oil matter, so be sure you've got your surface clean and dry before painting.
For masking, since you're working on oddly shaped surfaces, I'd consider trying something like rubber glue used in schools and offices. Nothing industrial like silicone, just basic rubber glue. Try a dab on an inconspicuous part of the metal you want to mask and let it dry for a couple hours to be sure it peels off clean. (Silicones often won't come off clean.)
Brush the glue along all the edges to be masked, let it dry before you paint and keep a hobby knife handy when you're removing the glue, to separate any spots where the paint hangs over the glue and might peel away. If you're spray painting, use masking tape on the major surfaces, then catch the edges with the glue. That should keep your tape from peeling up.
Good luck!
signs, graphic design - Monterey Park, California
2004
Very informative, sounds like a good plan. I think you're right about the anodizing. I shouldn't have to remove it b/c it is somewhat permanent and should hold up well. Good idea for masking as well, Thanks again
hobbyist motorcycle enthusiast - Palmer, Massachusetts
2004
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