There are basically two types of filler and you mask them differently, but for tanks with a raised neck and cap atop it what you do is mask the top, but not the sides. You leave the top of the filler tube, the part that the cap gasket rubs on, bare metal when the job is complete. Your paint job begins immediately after the gasket. Don't extend the primer, base, or clear over the top. What you do when you paint that type is immediately after shooting the last coat of clear you pull that tape. I mean immediately. What happens is the final coat of clean then flows down across the layer from the earlier coat that the tape line cut and seals it. Its outside of the gasketed area so gas can't get to it. If you let the paint end under the cap the gasket will rub through it pretty quickly and then the gas fumes will attack the paint and cause it to lift. Once that starts you're screwed.
The other type of cap is the recessed sort. You tape them essentially the same way, but what you do is tape the entire top and then take a razor and trim it back to the outside of the bulge which is the sealing ring. With these you can leave the tape after the last clear if you like. I think somewhere I might still have some photos of how to tape the two different types of fillers if you need to see it to visualize it.
Anyway, after you've painted a few and lost a few due to your own taping mistakes it sinks in how to do it right. All I'm trying to do is save you the grief and expense of doing it the wrong way.
