Brake caliper piston rusting from the outside in-how bad is this?

I was swapping pads and discs out on one of our 16 y/o Subarus today- the lower caliper pistons are seriously rusted from the outside. The uppers are about what you'd expect in a an older car.  The pistons compressed normally, don't leak brake fluid at the moment and seem fine, but this is some of the worst rust I've seen in a lifetime of dealing with Midwestern cars. I know the pistons are made of pretty thick metal, but do they ever actually rust through and leak brake fluid/ fail catastrophically?  We scraped the worst of the rust blisters off and coated the external surfaces of the pistons with caliper grease to slow down the corrosion. 

That's nowhere near a concern.  All that matters is that the chromed surface isn't pitted at the seal.

Maybe it's because I'm on the west coast and but I would replace that caliper just on principle.   Actually both on that axle.  How much can a couple of rebuilt calipers cost?   

In reply to SEADave :

I would trust this caliper more than a rebuilt. There is some serious junk out there.

Just buy new pistons and a kit of rubber parts and rebuild the calipers - easy to do, and not sure why everyone seems to go for calipers rebuilt by someone else. If you do it yourself you get the opportunity to examine and assess condition. 

In reply to wspohn :

Seized pins or bleeder screws, that's why.  Or just plain the caliper rusted real hard between the dust boot and the seal, and locked the piston in place.

I have seen caliper pistons way "worse" than that.  What you see is just cosmetic.

I've never heard of them rusting through, although I suppose theoretically it could happen. Looks like almost all of the metal is still there though so I'd have no worries about cleaning it up and running it.

Thanks for all the input folks! I  think I'm going with the majority and just sending these as they are... maybe will rebuild them or install fresh calipers the next time it needs front brakes. I just wanted a reality check to see if it was crazy to even think about reusing them. 

My concern with swapping out or rebuilding calipers is that it can easily spiral-- I'd definitely want to change brake hoses while I'm in there, then undoubtedly the hard lines would be rusted to the hoses, etc etc. 

I rarely change flex hoses with calipers, BUT if the tube nut is rusted to the line, you can just crack the nut loose first, then unclip the hose from its bracket, then push it down and thread the hose off of the nut.

I've seen inside of brake pistons that rusty. As said, they ain't gonna rust through in our lifetimes... as long as the chrome on the outside and the bore look good. If piston seal was leaking, of shows damage, put a new seal on. Otherwise, knock the loos scale off, and good to go.