I am test-restoring some brake calipers which I presume are steel and were zinc-plated some 30 years ago. see below for some details. what this question boils down to is how to know, after cleaning the parts, if the original plating is showing through or if the plating has been worn off by either scrubbing or corrosion. I am currently testing some moisture exposure to see if rust shows up. The dust that comes off is usually not helpful, being black crud or rust. I would like to avoid having to test it by boring a hole in there and look at the layers (or absence thereof). I would like to either clean them off and use them as-is, or proceed to a plating service or set up some zinc plating electrochemistry. I would like to avoid painting them if possible. update after some interesting comments : yes I know the bore/piston surface should not be plated, is the most critical area, but that is another question, as is the rebuilding or inspection of the bore and surfaces. again, this is about the - wait for it - original plating, and possibly by extension electrochemistry or painting - but even that is for another thread I think.
some particulars:
I scrubbed with brake cleaner, degreaser, goof-off, then started with the brass brushes. it managed to get the caliper very shiny in one spot compared to the insoluble black coating elsewhere. there is some rust, not bad though actually. the coarse brass brush on a drill worked best for bulk cleaning. the small rotary tool brass brush was OK for small areas but took far to long to be practical.
Brake calipers are seldom if ever plated…Typically, they are made of cast iron… As is ALWAYS the case, Make, Model, year, Mileage would be very helpful…
These are single-piston Ate calipers from probably a late MY Porsche 944. No idea how many miles. As stated, they are my test case. Again, this is specifically about the plating, not restoring them - that’s another post. The car I actually drive is 1987. Its got plenty of miles - even kilometers. The calipers are very rusty. At first glance, my car’s calipers and the spares are the same.
What calipers look like on the outside is unimportant. It’s what they look like on the inside that counts…Swapping brake parts between makes and models can get you into serious trouble unless you verify they are identical…If the calipers had any coating at all, I bet it was paint, not plating…
Calipers when new are coated with a cold zinc galvanized spray. And this is done just to prevent the calipers from rusting while they sit on the shelf.
Plating involves immersing the entire part first into an acid bath to clean it, and then immersing it into the different plating baths. This is usually copper, nickel, and then chrome if you want a chrome finish. Or you can skip the chrome bath and just leave a nickel plating on the part.
I’ve done, and still do at times, some zinc home plating and parkerizing. The process is a bit of a pain in the neck so my advice is to clean them and paint them with caliper paint if need be.
My situation is a bit different because I’m doing hardware and small motorcycle parts for which painting is not anatomically correct or feasible.
Plating a caliper is something I’ve never done so my expertise on that is zero. I would only advise that the caliper bores not be plated and how to avoid that would be only a wild guess on my part. It would possibly have something to do with the location of the zinc anodes during the plating process as the sacrificial zinc pretty much travels a straight line from anode to part.
Bore holes to see layers? The plating thickness, if any, is in microns. Unless you have a SEM, you won’t be able to see anything but a hole.
If it is already rusting in spots, what difference does it make? You have to go to bare metal to do anything that will last. After that you can choose any passivation method you want.
Selective masking is possible with all plating methods. However, it might be faster to simply hone the bores afterward than to mask them.
Like @Bing said, clean them up, paint them or powder coat (high temp either way) if you want. Don’t plate them, that could cause problems on the insides of the caliper, changing critical dimensions, maybe interacting with the brake fluid.
thanks for the interesting comments. I have adjusted the original posting to try focusing on the original question so this doesn’t derail too much, also because I will probably ask about bores/pistons/etc. later.
Inexpensive brake cylinder hones are available in different size ranges…
Critical area…The grooves machined in the piston to accommodate the sealing ring and the dust-boot. Those grooves must be in perfect condition or you must replace the pistons with new ones…You may find it’s cheaper just to replace the calipers with new ones than to “restore” the old ones…