How to remove markings from laundry sink
I had a similar problem after using a drain cleaner for a bathtub and for a sink.  I used Glass Plus, a multi-purpose cleaner found in the window-cleaner aisle of the grocery or big-box stores.  I sprayed it on, let it sit, and scrubbed it off with a sponge.  It is non-toxic.  

After finding that it cleaned a litter box and neutralized the urine smell, I sent the bottle and a roll of paper towels with the 7-year old boy to clean his toilet splatters!  Since the success if that, we have learned that we can put half water and half Glass Plus in the spray bottle, and we use it routinely to clean the outside of their toilet and the floor around it as well as their bathroom sink and tub.  Adding the water is just an economical way of making the Glass Plus last longer.  It is not expensive anyway, only about $4 a bottle, but there are 5 toilets in this house, with 6 people, 3 of whom are boys under the age of 13, who do not have specific aim when using the bathroom.  It is important that we use that spray to disinfect as well as to neutralize the smell of urine.  All our boys can clean their own bathrooms, and get sent to do it whenever they need to behave better.