I would be reluctant to apply anything that coats the surface, unless you aim for temporary cosmetics only. It may look good initially, but over time is bound to wrinkle, crack, peel off, get scratched etc.
Dyes or plastics refreshers like "Armor All", which seep into the matrix (somewhat), are the better approach in my view. They may evaporate or wear off over time, but can be easily refreshed. They may not cover up all stains or scratches, but at least they don't add further blemishes of their own in the long run...