Concrete cleaner. which is Muriatic acid (or hydrochloric, IIRC).
Stuff is STOUT. I have done many many motorcycle tanks over the years, and a handfull of automotive tanks as well. Those are fun (not).
Drain the entire tank. You'll need your muriatic acid. And a "Kreem" kit from a motorcycle shop.
Remove any sending units it might have. Fuel gauge? Take it out.
First rinse the fuel tank with water. Lots of it. Warm works better than cold (hot is nice too). Rinse it out real good.
Then grab a hand full of rocks from the driveway. I have a long gravel driveway so I have plenty of rocks and just perfect for stuff like this. Drop a bunch of rocks inside the tank with a little water. The water helps slosh the rocks around and keeps the rust flakes from getting under other rust flakes. The rocks bust the big stuff loose-if there is any. If it's just mild surface rust (and you can't see any flakes), the rocks aren't really needed.
Next step. Get some goggles and breathing protection, as well as some protection for your hands & arms. Muriatic can kill you if you try to drink it (hint). If you inhale the fumes, they will make your nostrils and airways all the way to your lungs feel like fire for a couple hours (experience speaking). You get it in your eyes, you'll be using a dog to drag you around. It's bad stuff---but for what we're doing here, it's the only thing that works. I usually use about 20%-25% (about 1/4) acid and 3/4 water. Soon as the stuff hits the rust, you'll know it. You will see steam and other bad stuff coming out of the tank. Slosh it around, all around inside the tank. bottom, top, sides, etc. Dump it into a plastic container and then treat it one more time. Normally 2 treatments are all that's required. If it's REALLY nasty, buy a new tank. The acid works pretty fast so you have to watch out...it "can" eat through the tank if you let it sit in there for a while at full strength.
Then you'll want to rinse it out REAL good about 2 or 3 times to get all the acid out. The last rinse, I like to use warm or hot water as it dries faster.
In the Kreem kit, there is some acid. Dispose of it. It's garbage. It works to brighten aluminum (or turn it black) but that's about it. The #2 bottle is a strong alcohol that helps to remove any water from the crevices. I use it, and all of it on a steel tank. Then dump it out. It also has some nasty fumes but not as nasty as the acid. Dispose of it carefully as well. It will evaporate quickly and the tank will get cold as it evaporates, and I usually suggest setting the tank in the sun while the stuff evaporates out. If not, it can draw moisture via condensation-and Kreem will NOT stick to moisture (which is why they send the alcohol, to help remove the moisture).
The last step is the actual tank liner. The "kreem" as I call it. It also has fumes; about like the alcohol. You will dump about half of it into the tank and slosh it around where it will coat the entire bottom of the tank. Once it's evenly distributed, dump the rest in and coat the rest of the tank, sides, top, everything. It takes a while and a lot of "sloshing"....but I tell ya, the stuff flat works. Done properly, the tank will never rust again. It is a plastic liner.
The kit itself is about $45 last I checked. The muratic is cheap. A gallon is like $3. You can buy 20% or you can buy the good stuff...the choice is yours. I get the good stuff because I dilute it anyway, and a gallon will last many tanks. BTW, dont get the acid anywhere near anything else metal that you don't want rusty. It cleans concrete REAL good too...and will streak it a little sometimes. The time it takes, well, on a typical 6 gal steel tank with a fuel gauge, it takes me about 4 hours from start to finish if I'm not hurrying...and you don't want to hurry. The liner needs to set up over a period of about 48 hours (the bottle says 24 hours...but IMO, minimum 48 hr). I usually like to do it in the morning so I can move the tank around & evenly coat the entire inside of the tank before the stuff sets up; then let it sit overnight and all the next day. Then it's done.
Given the chance, I'd buy a new tank...especially considering today's fuel quality. Fuel evaporates quickly nowdays. New tanks are designed to stop (or slow down) the evaporation process by not using a vent, or at least using a one-way vent. The old style tanks with vented caps WILL allow moisture to form inside the tank if it sits for more than a few days. Fuel evaporates, the tank and fuel temp drops (evaporative cooling), and moisture from the air condenses. New tanks without a vent won't let evaporation take place as fast. And with the price of a Kreem kit plus all the rest of the goodies to go with it, you're right back to the price of a new tank. I like the new EPA compliant Yamaha tanks. Lot thicker material and they don't swell much; unlike the Attwood I used to have that broke due to swelling....while I was fishing on the lake). Not fun.