What can I use to clean my black composite material kitchen sink?
A granite composite sink is made up of 90 percent granite, mixed with a resin. The resin helps to make the granite less porous and more durable. Unfortunately, even with the presence of the resin, the granite in your composite sink can still etch. Any products that are extremely alkaline or acidic can remove the finish from your granite composite sink. Always clean your granite composite sink as if it were made of 100 percent granite. Use a pH-neutral cleaner, or use a cleaner made for granite and stone. Using cleaners that contain citrus juice, vinegar or ammonia could strip the finish off the sink.
Everything I watched on YouTube recommended oiling the sink with vegetable oil, olive oil and even PAM and using a microfiber cloth to work it in to protect it. This product seemed to be the most popular for cleaning lime haze off it.... https://www.youtube.com/watch?v=xCe11RIjyls
I found an article on cleaning a composite sink that may be of interest. I hope this helps!