If it's just all chalky from the sun to the point you can't see through it anymore, buy a headlight restoration kit at any auto parts store. The latest have the little sanding discs that will fit into a drill and you work your way to finer and finer grit disks until you get to the ball and polishing compound. You'll use it on your car headlights anyway as they get chaulky and fade so it's dual purpose. I have one I've used to clean up my car headlights, polish the plastic faces on my boats gages and instruments that are baked in the sun, and even use it to clean the plastic tops on my solar landscape lights when that plastic gets so cloudy they don't charge much any more during the day. IF you go this route... Pull the cap/gage and restore it somewhere else away from the tank.. don't need a fuel tank exploading because you're polishing the fuel gage with a drill while it's attached to the tank!