How to clean inside of gas tank?

I sometimes do very old equipment where the tank can not be purchased. What I do then is fill it 1/4 the way with small stone. or nuts, bolts , screws, washers, etc. Depends on whats laying around.

Strap the tank to a old garden tractor rear wheel. Jack the tractor up and let it turn very slow so it tumbles like a dryer. If its smaller then the tire you can leave it on there to cut the grass. LOL

The slower the better though. It causes the tumbling action. The tank should be re-coated inside before using with a two part product. such as epoxy paint. Apply that the same way and dump the extra out.