Rubber in can from Alvin Products can be used to smooth over rough welds and can seal and protect any metal surface from rust. corrosion or water leaks. Rubber in a can is used by welders to seal duct work, a/c units and welded metal parts. Rubber in can can also be used on weld joints to create a water tight rubberized surface.
Every welder should have a can of  rubber in a can in their tool box and shop.