The rust can get down into the pickup and could cause the coil to break which would kill the pickup. At which time, it'll need rewinding or replacing.

While I have your attention I want to announce the release of an important new Kinman Strat pickup, the Impersonator 54. This pickup represents a major milestone in the evolution of hum-cancelling design for Fender genre single coils because it is the first to achieve non-aged sound.

Non-aged means scooped mids (high transparency) and brilliant highs with gun-shot attack and wide dynamic range where the sound does not break up under heavy pick attack.

The impersonator 54 is such a revolution because all existing noiseless Strat pickups with Alnico rod magnets (regardless of brand) deliver Aged sound where the midrange is more prominent, the highs are not as brilliant, the attack is softer and the sound breaks into crunch under heavy pick attack.

First let me explain Aged Sound with this rather interesting experience I had some years ago: a customer sent two 1964 Stratocaster pickups for repair, both were open circuit with broken wires in the coils. In one pickup the break was near the termination point and I was able to repair it without rewinding. The other one could not be repaired and had to be rewound, which I did using the original Formvar HF42 wire type. I wound it to the same DC resistance and tension which I judged from the repaired pickup. It looked identical and the resistance measured identical to the repaired pickup. But when I tested them in a guitar they sounded very different.

The one with the original coil which I repaired sounded dull and lifeless, it also had comparatively low output and the sound broke into distortion when I picked it strongly. The rewound one sounded just like a great NEW Strat pickup, bright, spanky, alive and loud and no matter how forceful the strings were picked/strummed the sound held together and did not break up.

With my curiosity aroused I measured the magnet strength and found them to be the same as a new pickup. I re-magnetized both pickups anyway but the sound did not change from either pickup. This experiment exploded the myth that aged magnets were the reason for this massive difference in sound. So I concluded it had to be the coil itself since there is nothing else to explain it. Another well known pickup manufacturer claims weaker magnets are the reason that old pickups sound sweet, but I can not confirm that claim when I deliberately degauss magnets.

To confirm my suspicion about aging of the coil I ran some analytical Q plots on both pickups, see the image below. To obtain absolute conclusive proof I also rewound the second (excessively aged) pickup. It then sounded identical to the first rewound pickup and returned an identical Q plot.

It turns out that Formvar insulation is not age stable, it’s an unsophisticated old technology coating that degrades over time, unlike modern Polyurethane coatings which seem to go on forever. Formvar is also sensitive to heat treatment so depending on the length of time spent in, and the temperature of the wax pot at time of manufacture, it took longer to degrade. That’s the reason some old Strat pickups aged more severely than others over a similar time span. Those with slight to moderate aging lost some ice-pick brittleness and sound sweeter than examples of new pickups. Those with excessive aging, like the one I repaired, sound so dull and lifeless as to be unusable, something like turning the tone control down to 3.

Does anyone have a picture of what would be considered severe rust?

but my vote goes to Richard Thompson's workhorse strat. That is a well played strat...

Yeah, but that would have to be an extreme case of rust. Almost all of my guitars have the pole pieces rusted or at least tarnished. It is almost unavoidable. I did take some sandpaper and knock some of the rust off of some pickups once, and didn't notice it harming anything...........I guess a fella could use a Q-tip coated with WD-40 or a light oil to swap the pole pieces each time you change strings or clean the instrument.....