I just read this - hope this may help someone with a current problem. Yellowing is generally UV deterioration and should not be happening unless the vinyl is not performing as it should. It most likely is also getting more brittle. Contact the manufacturer if possible and try to get a factory or distributor rep out to see. Many vinyl windows and doors have this problem when not formulated correctly. Acetone, as was mentioned, disolves PVC, and should remove the surface layer of yellowed material, but will void any warranty. It won't resolve the issue for the long run either as the material is the same all the way through and it will just repeat the process. Painting PVC requires special surface preparation, special paint with bonding ability and flex-additive (like auto bumper paints) or using vinyl dyes. These will also generally void your warranty and are difficult to apply and create maintenance issues for the future. Good luck with the manufacturer!!!