: My laundry room sink is plastic and I had the same problem. Wash the sink with Softsoap liquid cleanser and a nylon scrubbing pad. Rinse, then fill the tub with water to cover the stain and add a cup of Clorox. Soak the sink for a couple of hours (or more if needed). Then apply car polish to the sink and you won't have anymore stains. Hope it works for you.....it worked for me!