Review on Swify Fine Mesh Strainer Set of 3 - Stainless Steel Strainers for Kitchen Cooking, Tea, Coffee, Pasta, Frying, Vegetable Drain - Food Strainer Colander Sieve with Long Handle by Sandra Price

This set appears to be durable which is great. However, when I wiped it down with paper towels and rubbing alcohol for the first time, some of the dark, plaque-like stains were gone. What is that? At the same time, the products did not rust.———UpdateA black film remained after boiling in salt water. My rubber gloves turned dark after washing them with dish soap. Now I doubt the quality and I really hope the seller can give a reasonable explanation.