I presume that the tanks are not plastic but these are the instructions we give out with all our tank stickers -
*NOTICE: CERTAIN BIKES REQUIRE THEIR SHROUD GRAPHICS TO BE APPLIED DIRECTLY ONTO A PLASTIC GAS TANK. PLASTIC GAS TANKS ALLOW FUMES TO PASS THROUGH, WHICH CHEMICALLY REACT WITH THE ADHESIVE ON THE VINYL. WE USE A VERY "HIGH BOND" ADHESIVE, IN ADDITION TO CUTTING "SLITS" INTO THE PIECES BEING APPLIED TO THESE TANKS, TO HELP ALLOW THE FUMES PASS THROUGH. BUT THIS ALONE WON'T STOP THE BUBBLING EFFECT THE FUMES WILL PRODUCE. WHAT YOU NEED TO DO, AND NOT JUST WITH OUR GRAPHICS, IS FULLY CLEAN THE AREA WHERE THE GRAPHIC IS BEING APPLIED OF ALL PETROLEUM PRODUCTS AND SEAL THE TANK WITH A CLEAR COAT. FROM OUR STUDIES AND SPEAKING WITH OTHERS IN OUR INDUSTRY, THIS WILL HELP IN 90+% OF THE CASES.