How do I remove all old paint from the tank?

Use paint stripper. It's a mess, and takes a long time, but I striped an alloy tank on an old MX400B down to clean metal. I used a razor blade scraper to take the old stickers off, and different solvents to cut the glue (can't remember what worked best). I've tried different strippers, and found one at Lowe's called 'Dad's' that worked the best. I don't know if you'll be able to get it there though.

Original metal tank paint is an automotive type baked on finish. You probably can't do that. I wouldn't try any lacquer without spraying it on a sample surface and testing it with gasoline first. A friend of mine used some sort of clear coat on his daughters bike that ran off when he spilled gas on it.

I recently painted the tank of a $150 MX175 with Rustoleum industrial grade paint. It comes in a silver can for just over $5US. The color choices are more limmited than their other lines, but I thought the industrial stuff would take abuse better. Gasoline doesn't seem to ruin it.

if you wanted to just have someone sandblast it if you know someone with an automotive shop. and me personally i would try to find a place that does powder coating and have that done. at least with powder coating you are getting a really tough paint that can handle the abuse a lil better then rattle canning it. but all this would defeat the purpose of a do it yourself project. but just on a gas tank i guess i would use paint stripper.

might be the best $100 you've spent!!!