Well the problem with spray can paint is that it has no hardener or catalyst, they are usually lacquer or enamel. The only way I know how to prevent the damage caused by gas is to use catalyzed paints, like urethane or epoxy, these need to be mixed right before spraying then put on with spay gun. I realize this doesn't solve your problem but a local body and paint shop probably wouldn't charge much for just a tank paint.

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Richard-iRide Customs
"if it has tits or wheels it will give you problems"