As said earlier, with use of the pen-eraser gum - and certainly with the print-remover bottle that also is designed to remove varnish, at least according to the label - the plastic has become a bit dull. Not that much, but it is not as shiny anymore as it was.
So, first a bit of cleaning with a mix of kitchen-products (cleaners) and water.
Drying the plastic with a clean piece of cloth or with kitchen-paper, and in doing so rubbing good & long enough will (usually) already bring a bit of the shine back.