Unfortunately, neither of the Chem-Dip products that we offer is suitable for that application.  Instead, we recommend using Berryman B-12 Chemtool Fuel Injector Cleaner (part #s 0101, 0116, and 0121) to dissolve the varnish.   Generally, soaking for just an hour or two will saturate the cleaner with varnish, but the tank can be soaked overnight, if desired.  Be sure to avoid spillage on paint or other sensitive surfaces and plug any open ports to mitigate evaporation.  If practical, move the tank around so that the cleaner makes direct contact with the varnish.  Once the B-12 is saturated, the mixture should be drained and the process should be repeated until the tank is visibly clean and the product comes out looking essentially like new.