My 2 cents – NON CHEMICAL WAY.
I had a tank that was sitting for 8 years with gas.
1. Removed tank and petcock and cap.
2. Drain tank into a containter (take container to WASTE RECYCLER)
3. Power washed inside of tank
4. Dry tank with air compressor.
5. Seal drainhole with duct tape
6. Fill gastank 1/4 with a mixture of small sharp stones, aquarium gravel, BBs, broken glass, ect..
7. Duct tape over gas cap hole
8. Duct tape a thick towel around the gas tank.
NOW THE SCARY PART
9. Make sure your wife isnt home.
10. Stick tank in the dryer – NO HEAT – for an 50 minutes.
11. You may need to repeat steps 9 and 10 a few times.
12. You can also take it to a commerical laundrymat if you have a larger tank.
My tank was clean and shiny
For those of you that have a tank/frame – I pulled my tank/frame behind a boat for an hour with life jackets attached to it and a long rope. It came out great with no damage and the frame was really clean