This is a common problem compunded by the tire shop who mounts your tires if you pay to have work done. Their interest is in the waiting customer and to get to them you must be moved out of the work area..fast mount balance and out the door you go...it is not that they do a bad job, they just don't do a through job. They have no idea that the moisture that invades a tube tire over the years really affects the sealing bead area of your rim. Nor do they have the time to treat your rim for this contion due to the WAITING CUSTOMER. To correct this will be to dismount the tire..wire brush and acid etc, prime and paint the area...sandblasting is better but not all have access to that. If you find a lot of small pits after prime and paint, you can wipe a very thin bead of RTV on the sealing lip just prior to inflation..this will smooth out the illregualrities. go easy on the sealant..just enough to fill the tiny voids and not run to the inside of the tire...