Does anyone know how to remove hard water stains from a plastic sink

Line the sink with paper towels then dribble vinegar over them untill saturated. Let it soak for a half hour or so. If more is needed, make a paste of baking soda and vinegar.

Sprinkle Baking Soda in your sink and mist with vinegar. Let it set at least 15 minutes and then scrub.

When I was a kid, our house had a plastic bath tub. My mom always made us wipe it down with Vinegar first and then scrub it with baking soda. I don’t remember how well it worked but my mom still swears by it.

This process requires sun light so start early in the day or use a UV light. 1)Clean the sink with denatured alcohol, let it dry. 2) put on gloves and eye protection. 3) spread 40 Volume Creme Developer (get it at Sally Beauty Supply Store) on the entire sink with a tint brush. 4) Cover with cling wrap and let it sit in sunlight. Try to keep the layer of 40 Volume Creme Developer even to prevent streaks. If you do get streaks you can just repeat the process. Most plastics respond to this bleaching within 1 sunny day.

Baking soda and vinegar has worked for me too- sometimes a magic eraser will help with those tougher stains- good luck :)

Barkeeper's Friend or Ceramic Hob Cleaner.

Is the sink acrylic? I had one and after time the protective coating can be scrubbed away and when that happens you need to use bleach and baking soda. I use the baking soda to hold the bleach in place on the stains. Apply and let sit over night and rinse the next day. But this only works for a short amount of time.