i have 4 stingrays and the same problem.i play a lot of hours and the pickups have a dirty.how i can remove the rust from the pole magnets without damage the pickup and the sound of my basses.thanks
Rust on the pole pieces, at least if it's only a bit, will not affect the sound of your bass. I agree that it can look bad, but unless moisture gets inside the pickup itself, it's not a big problem.
You want to be extremely careful if you decide to try to get the rust off the pole pieces. If you use something metallic or sand off a small bit of the magnets themselves, you'll never get all the metal filings out of the pickup and that WILL cause real damage.
I'd either leave it alone - if it's not affecting the tone of the bass - or, if you have noticed a difference, then replace the pickup.
You might be able to get some of the rust off with WD-40 or similar penetrating oil. Use small amounts on a cloth and wipe it on the pole pieces. You might want to repeat that a few times. The WD-40 won't harm the pickup or any other part of your bass, but you won't want a slick mess if you aren't careful. It may not get all of the rust off, but it will help prevent it from getting worse if occasionally applied. It's best to occasionally apply some to the pole pieces before the rust can begin.