Rusty caliper pistons - clean up and use or toss?

I snagged a set of FD RX7 brake calipers for a good price. I was told they were dirty and a little rusty but for the price it was a risk I was willing to take. The caliper bodies are aluminum so that "rust" was just staining. I'm confident I can reuse the bodies with a OEM rebuild kit but I'm concerned about the pistons. The surfaces that are normally inside the bores of the caliper are fine - no rust, scratches, etc. It's just the "top" of the pistons that are in questionable shape. Here's what they looked like after soaking in WD40 for a week:

Today I took Dremel brass and carbon steel wire wheels to the best and worst examples at low speed. This took off any remaining scale and all that's left is the pitting:

Best on right. Worst on left. Their appearance doesn't bother me and a little surface rust where the piston contacts the pad won't be an issue. The thing I'm concerned about is the pitting on the "neck" of the pistons. This is where the dust boot makes contact and I'm worried they won't seal well with the pitting. My next plan of action is to soak the affected areas in Evapo-rust to eliminate any remaining rust then find a way to chuck them to my drill and try polish out some of the pitting.

Am I wasting my time? New pistons are available through Mazda Motorsports at ~$25/ea so replacing all 8 would be over $200, which is more than I paid for the calipers. So far I haven't found any equivalent aftermarket pistons (36 mm Sumitomo).

Perhaps a high temperature rated filler?

The dust boot also seals against the underside of the piston lip so I wouldn't be too concerned. The critical area is where the tape is which is where the piston seal contains the brake fluid.

Yeah the actual sealing surfaces don't look bad. You can bring back some surprisingly bad pistons by turning them in fine-grit wet sandpaper.

As Gameboy said. The area you have taped is the only important area.  I would be delighted if those came across my bench.

In reply to Trent :

+1. Those pistons are fine. The dust boots will still do a perfectly adequate job of sealing around that pitting.

Those look better than the ones I reused on my Porsche...

Thanks, all. I figured they'd be usable since the sealing surfaces are fine. I've been soaking the crusty areas in Evapo-rust all weekend. Follow-up question: could I zinc plate the pistons to discourage future rust? Most everything I've read says the plating thickness is small (<0.001 in.) so it shouldn't affect the caliper's function, I'd think.

The important sealing surface is hard chrome, like a hydraulic ram. Do not zinc plate over that as it is the working surface.

You can mask it off with electrical tape and plate it if you want to but a shot of spray paint will be sufficient. 

Unless you are plating at home the cost of zinc will be similar to the cost of replacement pistons

Sounds like masking and plating only certain areas would be a pain, or expensive. I'd considered paint. I already have the sensitive areas taped off so it wouldn't be hard to hit the previously rusty spots with some high temperature paint. The only reason I bring up plating is because the caliper hardware is in about as good condition as the pistons and I was looking into getting all of it plated.

In reply to infernosg :

Totally. I am a big proponent of plating. 

Even the calipers for my Rabbit pickup have been nickel plated. 

And the hardware for the Fiat's calipers as well.

I am an "If in doubt, plate it" guy. I still wouldn't do the pistons though. 

infernosg said:

Sounds like masking and plating only certain areas would be a pain, or expensive..

Look into Cold Bluing.

In reply to Trent :

What kind of plating did you go with for the hardware? Typical zinc or something fancier? Looks like there may be a place near me that does black oxide (no), tin, zinc, cadmium and more. I know cadmium is supposed to be the best but with all the EHS stuff I'm betting it comes with a cost premium.

In reply to L5wolvesf :

I'll admit I never thought of that. I'm familiar with the process through other hobbies...

Those parts are nickel plated. My local folks do zinc and nickel. 

Zinc and cadmium are "sacrificial anodes" so they will deplete over time depending on the elements they are exposed to. Nickel is protective in that it is a more robust surface that doesn't oxidize.

I just think the nickel is pretty.

My mini calipers are zinc with blue chromate. When I do porsche calipers I go back to yellow zinc. 

In reply to Trent :

Thought I replied to this. Anyway... Did you go with nickel plating or electroless nickel? I have local shops that claim to do either. Some quick online research says the latter may be better for a more uniform finish. What concerns me about nickel is I've read it's thicker than zinc or cadmium. Clearly you've plated hardware so I'm guessing you haven't run into any issues.

Electroless

I use these fine folks

https://qualitymetalfinishing.com/processes-and-services/electroless-nickel-plating/

In reply to Trent :

I may have to give them a call. Every local shop I've called that advertises metal plating seems to strictly do large commercial/industrial jobs. I've got ~30 bits of brake hardware that could all easily fit in a USPS small flat rate box...

Trent said:

Unless you are plating at home the cost of zinc will be similar to the cost of replacement pistons

What is the cost of parts that you cannot buy?