Jeff, painting is not a long term solution. If you are using the tank and fueling, eventually it will develop micro cracks in the paint and allow fuel and then bubbling in the paint. I do a lot of painting, and we've had the "cross linked polymer" and HDP conversations here before.
On the internet, you will find several sites and videos that illustrate using "saloon care" hair product to get yellowed plastic back to white. I tried this last year, and it worked pretty well. Not perfectly, but good enough that the old tank did not look too bad on a freshly painted arctic white urethane painted Tecumseh engine. The stuff is called "Saloon 40" and you can find it on Ebay. It's cheap and you don't need much of it. Apply it thick, the cover the tank with a plastic bag, (clear) or plastic wrap and leave it on the sun. The UV acts with the peroxide to bleach it white without damaging the plastic.