Master cylinder....paint or powdercoat?

Thanks for the confidence gentlemen but I have only coated a few master cylinder covers thus far. Cast iron does turn out beautifully though and is a favorite substrate to powder coat.

My concern would be centered on the internal seals more than anything. Assuming you have a full rebuild kit and mask the cylinder housing well enough, with proper prep and curing, and keeping any spills cleaned up promptly I see no reason it wouldn't last.

Dot 3 brake fluid is supposed to be caustic. I experimented years ago with a freshly coated bracket submerged and sealed up in old nasty Dot 3 for months with NO adverse effects at all.
Dot 4 or Dot 5 silicone brake fluid will not hurt painted or coated finishes but require a full system drain-and-clean first.

If I was doing one for a customer I'd be inclined to mask not only the interior surfaces and plug the inlets but also about a quarter inch of the exterior too (where the gasket and lid mate to it) just to keep any brake fluid contact to a minimum so the customer wouldn't have to worry.

I hope this helps!