plastic tank repair

I had a leak in a camper's polyethylene water tank 20 or so years ago (pre-internet). Tried several products that seemed like they ought to work, but nothing seemed to stick to the tank. Finally, someone told me that welding was the only option.

It was a small crack near an embossed trademark, stamped in the side. I used a big soldering iron, and ran it along the crack, adding extra plastic shaved off a fitting on the side. It wasn't pretty but it sealed and lasted for several more years, before I sold the camper.