dirty fuel tank

Well my fifteen is alive after sitting for 25 years out in a field. And a couple hundred hours of work. I started it last night using a makeshift fuel tank. Now I need to clean the old tank. The tank is in great condition except for all the tar like varnish in the bottom. What works best for breaking down the crud? In the past I have just used gas or diesel. Does anybody have any thing that works better?

The "new" gas with alcohol does a good job of cleaning fuel systems. .when it came out, lots of folks wound up with clogged fuel filters. I'd give it a try first. Other solutions might include a soak in a hot tank or steam cleaning.

I have used acetone and MEK for the tough tar deposits. Be careful with these, and use outdoors with good ventilation, very flammable. Jim.