Once you clean both the poly chime and the part of the keg it came from, you need to rough up the stainless with a bit of fine sandpaper. Make sure everything's dry, and use a can of spray adhesive from 3M. I think this spray's normal use is re applying rubber coving in offices, or fixing delaminated corners of plywood. It goes on looking quite thick and rubbery, but everything sticks so well after its on. Either way, this suggestion came from a former brewing assistant at my last job, and it worked wonders for getting the kegs back into rotation.