I'm not sure about Chemtool or SeaFoam, but varnish and such is exactly what carb cleaners are made for. Something like Gumout, Pyroil, Holley, CRC, or similar products would help dissolve any varnish-ey, gummy residues in the tank. (I'm thinking liquids, not aerosols) Some of them are meant to be mixed with gasoline to do their work, so read the labels. If you can dump some in there and swish it around to all the surfaces a few times with the tank closed up, that should help loosen up the gunk, then I would think regular filling with gas would wash the stuff away. You might want to use a good carb/FI cleaner like Chevron Techron (PEA), Berryman, SeaFoam, Gumout, 3M, etc. in the first few tanks to add the extra detergency to dissolve the remnants. I've never really had to do this before but varnish and gummy stuff isn't really that tough to get rid of. It's not like tough cooked-on valve deposits, ring crustiness, etc. The varnish mostly dissolves away on its own with use, but the cleaners might help get rid of it and suspend/burn it better than just straight gas. Interested to hear what others have to say on this topic.