13B, rust on plugs?

A couple days ago I swapped the plugs on my FD, the old ones looked like this:

Going left to right, that's the leading #1, trailing #1, trailing #2, and leading #2.

The leading plugs appear appear to have some rust on them, which is a bit worrying... Any thoughts? This is the original motor in the car, 58K miles. I bought the car last year with 55K, so clearly it's spent a lot of time sitting, not being driven. I don't have compression test numbers on it -- had it inspected at a Mazda dealer before I bought it, they said "compression is good", but didn't give me the numbers (or if they did, I lost them). It's also unclear if they did the test hot or cold.

(build thread here)

Speculating wildly... Coolant seals with a slight leak?

Could be inner coolant seal, although on mine it only affected one rotor. If it IS a coolant seal the general consesus is you want to keep running the engine until you're ready to replace it. Otherwise you risk rusting the irons.

I think it is more of a residue from the fuel being used.

Any water getting into the combustion chamber would turn to steam and evaporate.

I think it's a symptom of sitting around too much.

I know little of dorito engines but if it was sitting a lot I'd be inclined to just blame atmospheric moisture and keep an eye on it. Same as how when you leave a piston engine sitting long enough moisture inadvertently works it's way in and the rings rust to the bores.

Yeah, the car has definitely sat a lot -- it averaged 2200 miles a year for most of its life, and probably got a lot less than that for the last decade before I bought. It came with full-depth AS tires with a 2007 date code, and the muffler had rusted through from the inside.

It's got new plugs in there now, I'll run them for a couple weeks and see what they look like. I'll probably stick a USB inspection camera in the plug hole when I check them again, as well. It was losing coolant when I first got it, but I found a couple of leaks (AST and water pump) that I fixed recently, so time will tell if there's more than that to it.

I guess my main concern was if this was a "OMG, stop driving it now before you wreck the rotors" kind of symptom, but it sounds like that's probably not the case. Thanks. :)

A lot of fuels and oils made reddish looking crud. Don't worry about it.