1981 yz60 stained gas tank restore

I have an 81 yz60 i'm trying to restore .All the plastic was scratched and faded. Ive sanded and polished the fenders and side panels with descent results.The gas tank is a much darker yellowish brown color. Its still the original color under were the decals were. Not sure if this is gas and sun combination but no matter what i do the tank looks the same. Any ideas, secrets? Ive seen many older yz restore projects that look brand new. Help please.