I've got a ~5 gallon plastic gas tank on the garden tractor, used for mowing spring/summer and has a blade for snow. It has two hairline cracks that I have tried to seal by "welding" the plastic with a small flat iron attacment to a soldering gun - no joy. A new tank is $65 so I'm thinking of an internal sealer. Red Cote says not for plastic or fiberglass. What's a good sealer that doesn't break the bank??