I have cleaned numourous motorcycle tanks using this method.
Poured some carb cleaner in to get the varnish out. Slosh it around for a while, let it sit and then slosh it around some more.
If there is rust or heavy crud in the tank, I raid my 100 years drawer for some nuts and bolts. I fill the tank about half way with hot soapy water and the nuts and bolts seal it up and start rolling it around in on the nice grassy lawn (I like the tractor tire idea better). I then drain and flush the tank. If I'm going to treat and seal the tank I go ahead and do that. If I'm just going to return it to service I pour in some wd40 and slosh it around to prevent flash rust before the fuel gets in it.
