The yellowing process can't be stopped unfortunately even when the plastic has a grey coating.
Look at the photo.
A piece of Super Nintendo’s case was chipped off accidentally at some point in the console’s recent history. Notice how the center of the case’s plastic — which hasn’t been exposed to air for very long — is still grey.
Sweat,dirt,grease from your hands, UV and oxidation cause the chemical reaction with the plastic to yellow. The ultimate protection is to seal your unit in a lead-lined, temperature-controlled, evacuated vault away from any air, radiation, visible or otherwise.