Tire Bead Sealing
My E34 had factory chrome wheels and when the chrome started to crack and peel, they leaked. The tire shop cleaned the bead as best they could and used a "sealer" of some sort to get a leak free seal.
On a parts car I have, the steel wheels were corroded pretty badly and I had some junk tires put on the wheels just to be able to move the parts car around. When they installed the junk tires, the tire shop boys said there's no way a bead like that would hold air, so they put the sealer on those (looked like black RTV) and the parts car has been on inflated tires ever since.
Link to my M50 engine rebuild thread
http://www.bimmernut.com/forum/showthread.php/43339-M50-Eng-4L30E-Trans-RRR-Far-More-Than-I-Ever-Expected?highlight=93+525+paul