Over the years I have read and heard about a variety of homebrewed methods to clean an old gas tank of scale and rust. Some have involved link chain, sheet rock screws and some other rather bizarre combinations. There are a variety of chemical kits out there but they tend to be a bit pricey.
The method I have used many times over the years very successfully is this one. It works for me and feel free to give it a try.
Supplies [A] 5 pounds of aquarium gravel and [B] 1 quart of laquer thinner or Acetone
Here's how it goes. Depending on the size of the tank get an appropriate amount of aquarium gravel. Basically motorcycle tanks need less than a 26 gallon truck tank. The best gravel has the sharp edges not the rounded off river style. It is small enough to get into all the nooks and crannies but comes out easily. I use a 5 pounder for most auto gas tanks. It does the scrubbing. The laquer thinner / acetone are the cleaning agents. Feel free to adjust the amounts of both as you see fit.
STEP 1 - Once you have the tank out and stripped of all the mech and electrical stuff pour the gravel into the tank. Add in a quart or so of lacquer thinner or acetone. Seal all the openings up. Now we get to the hard part. By whatever method you chose, shake, rattle and roll the tank. The more the merrier. Big tanks are best done with two people. Could look a bit wierd to the neighbors but what the hey! Make sure to do this in all directions. Including upside down.
STEP 2 - Once you have tired yourself out sufficiently unseal the tank and pour out the stuff. If it is a normal tank your going to see some discolored fluid and possibly rust and scale. After getting that done you can take the tank and flush it out with a garden hose. Any left over gravel should come out with this flooding. A quick look inside should give you a good idea if the scrubbing worked. It is not going to be new shiney but should be much cleaner. You may choose to make a second pass but I have never seen a need.
STEP 3 - Blow out the tank and let it air dry. Your Done! Reassemble the clean tank and sender / pickup.
Now, there are kits out the to coat the inside of the tank. My only words of caution are that many of these kits have a warning about not being compatible with some ethanol or other fuel types. And mix fuels are becoming more prevelant. My personal take on this is that if the tank is solid and serviceable don't bother to coat the inside. Why run the risk of a problem. Go ahead and paint the outside but don't coat the inside just for the sake of doing it. If the tank is bad you really should replace it anyway. Then this whole procedure is a moot point.