I used the new fusion paint from Krylon. Start with a clean dry tank. I used a degreaser on the inside and a very light sanding on the outside. I used the whole can and focused on the portion of tank exposed at the filler / seat area. (The part that shows if scratched.) After paint had set for a full week, I tried it out. Gas in the tank doesn't bother it, but if any gas gets on the paint, it will wipe off. The heavy coat of paint helps here. In order to get a good "plastic" look, wipe the exposed portion of the tank with 00 steelwool. It will give it a dull finish to match a newer '03 -'04 black tank. Also, a quick wipe with the steelwool will camoflague any scratchs it picks up in the future.
What ever you do, don't try to apply a touchup coat of paint over your job at a later date. It will blister in tiny little bubbles that, when wiped off, gives the bike an apperance of surviving a severe case of acne. If you deside you don't like the paint job, gas will remove it completely if you wish. Be safe using it though, it is gasoline afterall.
Hope this helps.