Sun-proof treated lumber sealer to retain natural look?

Pressure treated southern yellow pine picnic table. Was left outside in the package for a couple months (basically wrapped in cardboard with the ends sticking out). Only about %15 sticking out of the cardboard package was weathered and grayish. Just put it together. Planning to sand away the gray areas to look new like the rest and then seal it. I haven't read product labels or anything about sun-proof sealers yet if any exist. Looking for a one-time application if possible. Considering just staining it with something like minwax also. Question is if anyone happens to know of the best sealer to use that will keep the elements from changing the color? Also, would it be a bad idea to only seal the top showing parts and not the underside etc? Because I think this will allow the wood to breathe and not seal in moisture, but I'm not sure if having the underside unsealed/stained will do more harm than good. Thanks for input.