I have the Harbor Freight plastic welder, and it didn't work for me on polyethylene. The heat was very hard to control, because the temp is constant, and air flow is all you can change. It tended to blow the molten plastic around on me.
Instead of fighting the air welder, I went to Urethane Supply and bought the KC Welder Pro. It's been a terrific tool, and I recommend it. There's an explanation of how it's used to repair a kayak on the site here. I don't have a kayak, but it worked great on my polyethylene dash.