whats the best way to get old varnish out of a tank?

Thanks for all the advice.My friend and engine builder owns a machine shop here in town, he said we can hot tank it.I plan on replacing and changing the sending unit and fuel line. I think I would like to coat it, just to be sure.
I like to do stuff myself....so if I end up needing a new tank, I will have everything else new already.LOL
Andrew/Kidd