When I construct tanks, I alternate between a 50/50 mix of acetone and lacquer thinner, and a 50/50 mix of soapy water and alcohol. I usually do 3 cycles, shaking vigorously. The final flush with fuel.
If it's an old tank where you suspect rust, and you think you have to use it (like it is buried in the fuselage), you can use a plumbing "rust dissolver" solution. Not like naval jelly, this stuff mixes with water and you allow it to soak. Get it at the hardware store. Flush it with water/alcohol, then fuel or WD-40 and then fuel, to displace the water.
One trick that used to work was to leave the tank full of fuel over the winter, but I haven't done that in a long time and have had no issues. Leaking while sitting there for months can be a far bigger problem.
Brett