I'm suprised that anyone has had luck with painting the tank! My experience is that gas tanks emit powerful fuel vapors through the plastic and NOTHING will stick to the tank for any length of time. Graphics are small compared to overall tank size and even they won't stay good for long. I'd go for a new black tank if its in your budget, although money's tight for all of us.
may try an inexpensive black vinyl sticker material from your local graphics shop. It stretchs nicely with a little heat and is very cheap to replace if it bubbles up. Blah, blah blah! I know, but hope the info helps. Sorry, tend to ramble on and on. Oh, by the way..... Just kiddn'.
Marty aka maddoggy