I've tried it in the past Jerry and never had any luck getting them to seal 100% afterwards. If I'm just doing a quick fix, I seal them up with an automotive FIPG (form in place gasket) compound that is used for sealing oil pans these days. Otherwise replacement tanks aren't too awfully expensive. It hurts less than the headache I got from trying to weld them.
YMMV.
YMMV.
Willie