I got a fiberglass tank from ebay yesterday and started to tear into it before I saw this thread. Its rattlecan painted yellow with some decals on it. What I did before I gave up was use gas, as there was already some runs, and brake fluid, cause you figure you'll spill that on a tank at some point (at least I will in my clutzy ways). Well, long story short, nothing came off, not even the decals. Hosing it off after the fact to get all the caustic chemicals off, I again tried to get the decals off. And they peeled off without any issue, without anything but water....figures...I'll let the paint shop handle priming the tank and sidecovers from this point on...