I have had plastic kitchen and bath sinks in both my trailers. My sinks always look brand new. I use sinks mats in the kitchen sink to prevent scratching. It does get stains, especially coffee stains in the sink, under the mats that were hard to get off before I found my trick. I use Turtle wax "Polishing" compound. It's a white paste, not wax. I apply that like waxing a car, let dry and buff off with a soft cloth. The sink comes out bright white no stains and shiny, but it is not waxed and will stain easily so I then use a spray wax on the sink to finish. Works well for us.