Re: Cleaning out Old Gas Tank
Pour out all the old gas, in a safe manner that won't hurt anything

and pour in about a quart of denatured alcohol and swish it around. If there is rust in the tank, take a good hand full of hex nuts or pee gravel and add that in and shake it around until your arms feel like they are going to fall off

Pour out the resulting liquid and repeat as many times is need to clean the tank. After it is all clean inside, rinse out the tank with water and stick it in the sun to dry out for a couple of hours.
As for coating the inside, there are kits designed for coating motorcycle tanks that work well, just read the directions and follow them to the T.