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I have a great cast iron skillet that I use frequently and also take superb care of. Until I noticed the bottom is coated in rust. I believe the high heat that I use from the range top cooked off all the seasoning on the bottom + I never re-season this skillet ever since the business end of the skillet is in such fine shape.
What should I do to repair the bottom?
Get the rust off completely (wire wool or even the plastic equivalent will do), and lightly re-season (thin coat of oil, then cook it on the stovetop--you don't need a "real" seasoning, which will just endanger your interior if it's already good).
Then, don't depend on the seasoning to protect the pan. Always dry it thoroughly before storage (towel dry carefully then give it just a few seconds on the heat to be 100% sure) and make sure the place it's sitting is always dry as well. You might consider placing it on a paper towel or other absorbent surface if you feel that the place you store it can't be kept sufficiently dry, or if you're storing it on a metal surface (to eliminate the chance of galvanic action causing the rust).
If you never leave water in contact with your pan and keep it from being in contact with exposed metal, it won't rust in normal use. A little bit of seasoning on it is a little insurance policy, but as you've seen, it's not sufficient by itself.
It's not really a problem (in the absence of water, it won't get any worse), but if it bugs you you can scrub it with some steel wool and put some oil on it (be careful if you use something flammable). Generally the bottom of the pan doesn't stay seasoned (about half of mine are enameled on the bottom, so that wouldn't even make sense.) (I appear to be addicted to parentheticals today.)
The most likely cause is it not drying completely: if I have to put water on cast iron cookware, I always put it on the stovetop on low until it's completely dry.
