Hey Chodette, for your gas tank the old school way works wonders (imo).
Put some c-clamps on each side and hang it up from a branch or even a cherry picker.
Find some small gravel, like what walmart sells in the garden section. The sharper edged gravel is better. I just use some of the shale from my driveway.
Put a coffee can or two in the tank and a can of chemtool b-12 in, and start shaking. Work the hell out of it and make sure to turn it over a few times. Then dump it and do it again if you want.
You will be AMAZED at what comes out. As for tank lining, ive never used any, and I've had my toys for a long time. I have heard that they are good though.