Seal it from the inside with URETHANE, not silicone or butyl tape. Sealing it from the outside will only make life a living hell if u ever need a w/s replacement. My question would be why is it leaking? Was it ever changed before, and if it was, is it a shoddy bead or is there rust? If there's any rust, you can reseal it all you want and it's not gonna matter.