That varnish will need a stronger solvent than gasoline to remove it. Lacquer thinner or acetone would work, or a strong solution of Trisodium Phosphate in HOT water (concrete cleaner) would be another option. I've cleaned gas tanks by putting a couple of shovels of pea gravel into them, and strapping the tank to a rear wheel of my tractor. A couple of hours of bush-hogging weeds in a pasture does a really good job of cleaning.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!