The bead is the tire edge of the opening. It mates to the bead seat and seals the air in. If that seat of the rim becomes rusty and pourous it can leak. What I have done was mount the rim on a mag wheel machine clamping it by spreading not viseing. This way the clamps are not in the way. I use angle grinder and a 80 grit pad and grind it as I spin it. If it's real bad then it may not be worth it, but I never seen one it did not work on.
