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muriatic is my favorite method, especially if you are trying to maintain the paint on the outside. I just took my tank to a radiator shop because i want the paint off it, when they get done the paint is totally ruined or completely gone. They are charging me $50 just to 'boil' it... he said 100-150 if i wanted him to weld up the holes and coat it, but i'm going to do that myself.
The muriatic acid is fast and cheap
1) make a secure block off plate for the petcock. If you have some plexiglass laying around it works good for this. I've used corks, rubber, all kinda wierd junk. Anything aluminum or zinc (the petcock is made out of zinc) will completely dissolve in seconds.
2) pour in the gallon of muriatic. fill the rest of the tank to the top with water
3) drink a beer or 5. The concentration of the acid and the crustiness of the rust determines how long it will take to work. A 2-3 gallon motorcycle tank mixes 1 parts acid to 1 or 2 parts of water and takes about 2-5 hours. Watch it closely, its extremely reactive and it will eat a hole in your tank if you don't stop it once the rust is gone.
4) dump the acid into your neighbors' flowers. or better yet use it to clean oil off your driveway. it will literally dissolve concrete so be careful.
5) rinse the tank.. like crazy. Take the block off plate off of it and let a garden hose run in it for like 20 minutes pouring out the bottom and overflowing the top.
6) wash it out with dish soap water solution. the dish soap will get out any remaining greasy gunk and its a base that will neutralize the acid. Squeeze a little dish soap in there and shake the tank with water in it
7) do the rinse thing again. like crazy. drink another beer and let it run for 20-30 minutes
8) fully dry the tank by sloshing around some denatured alcohol.
--- this is where you would do the coating, follow the manufactuers instructions, i prefer caswell but usually I don't coat my tanks, only when they are pinhole leaky or really rotten and the caswell can actually fill the pinholes and seal them up. ---
9) put the petcock on, gas cap on, fill the tank with a mix of 2-stroke oil and gas. i usually do a good slosh of oil (maybe 4-5 ounces) with about half a gallon of gas, slosh it completely inside the tank.
10) put the tank on the bike and promptly fill it the rest of the way with fresh gas. you can leave the oil in there it won't hurt anything.
my theory is that having oily gas sitting in the tank helps oil soak into the porous metal and protects it from ethanol gas that can hold dissolved water. the ethanol gas dissolves water and oil, but the oil bonds better to the metal. Keeping the 2-stroke premix in there for the first tankful protects the metal.
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