Here is a copy of a previous post.
These instructions were originally written for someone with a "Tank in frame" moped so if you have a removable tank, some operations are a lot easier.
There have been posts on the forum refering to the use of gravel, BBs, nuts and bolts and other abrasive odds and ends which can be shaken up in the tank.
The only kit I am aware of for doing this is a Kreem kit, it is quite expensive but very good. It is difficult to use on a tank in frame ped.
I have used muriatic and phosphoric acid. I prefer phosphoric because it does not promote rust. Brand name cleaners have been mentioned on the forum which contain phosphoric acis such as CLR and Naval Jelly.
I would stick with the acid only approach, as opposed to buying a Kreem kit which coats the tank. If your tank is excessively rusted, to the point where gas is seeping through the rusty spots, go to a motorcycle shop, buy a Kreem kit and follow the directions. The coating will seal small holes.
Pick up a gallon of Phosphoric acid and a bottle of "dry gas".
Most buisinesses using large stainless steel vessels use Phosphoric acid. Dairy farmers, dairy products, brewerys etc. Posibly you could call one, or just stop by, it is cheap and they may just give you some. I have also heard that it is sold in Meijers pool section so I suppose other pool supply stores would have it. "Dry gas" is available at auto parts and gas stations for removing water from your fuel system.
Whem you use it, you are going to have to figure out how to get it back out of the tank safely. Typically, with a removable tank, you can just duct tape over the holes and dump it by removing the tape. You must remove the petcock and removing the gas cap is a good idea as it has a vent hole which can leak acid.
If the petcock scews into a threaded hole in the tank, maybe you can find a brass nipple that will fit the thread and allow you to drain the acid safely. If you have no choice but tape, remember that a running hose is your best defense against disaster.
For all of the following, keep a garden hose available and turned on. Wear old clothes, there is a good chance they will become religious, as in holey. Work outside, away from anything which could get splashed. When rinsing, use a lot of low pressure water, high pressure will send acid to places where it may do damage before you find it.
Dilute some acid in a plastic container and test it with a piece of rusty steel. You want it to remove the rust in about ten minutes but you are not looking for a violent reaction.
Remove the petcock and plug the hole with something appropriate, good duct tape usually works. Remove the gas cap if possible, if not, cover the hole with saran wrap before closing the cap to keep the vents from filling with acid.
Pour enough diluted acid in the tank to almost fill it. Seal the filler with tape or the cap/saran wrap and slosh the acid so that it is cleaning the top side of the tank. Just rocking the bike will do this. Keep an eye open for leaks and rinse off any acid you see immediately with a slow hose. Keep sloshing every minute or so until your time is up as determined by the test above, then slosh and wait five minutes more.
I am not sure how you are going to drain the tank but if you have not figured out a drain tube, lean the ped so most of the acid will fall straight to the ground and keep a sready flow of water on the hose to rinse any parts of the ped which get splashed.
Btw, letting the acid run into the ground in your yard is not an ecological disaster. Many plants prefer acidic soil. Don't let your pets get near it, dilute it a lot and don't dump it all in one place, especially not at the base of a tree.
Check the inside of the tank to make sure you do not need another application.
Using a rinse and slosh tecnique, get rid of all the acid in the gas tank then dry off the ped. If you have an air line, blow out the tank. If not, dry it as well as you can with rags, paper towels, hair dryer etc. Then replace the petcock and pour a bottle of "dry gas" into the tank and close the lid. Slosh this around for a few minutes to mix with any remaining water trapped in the tank seams, then drain it out.
Add gas and ped away.