welding is out of you want to preserve the chrome. Id suggest to do a mix of the above, id use masking tape on the face of tank and burnish it on good and clean so it acts as a moat/mold id use either epoxy or pc7 / jb weld from back side. personally, id just use jb, clean the inside of loose rust and a little scuff. mix the jb put it on from inside, wait for it to start to kick and slice off excess with a razor blade and be done. jp will be lest noticeable, and bondo or kitty can and will pop, plus bright colors. they sell that aluminized repair putty at all hardware stores.