Aluminium causes grey marks on items that it rubs up against when the metal starts to break down. These marks do not come off easily, and after a normal cleaning they are still there. The main place these marks occur is in the dishwasher when aluminium pans bump against dishes, but it can occur in the sink as well. If you have grey marks on your dishes or porcelain sink, you may be able to remove them with a few simple steps.
Apply a small amount of mildly abrasive household cleaner to a damp rag. The abrasion will help rub the grey mark off. Do not use a harsh abrasive, which may scratch the dishes.
Rub the rag on the grey mark in small circles until it is gone.
Wash the dish again to remove the cleaner.
Dampen your sink with a little water and sprinkle baking soda over the grey mark.
Spray window cleaner on the baking soda and wait half a minute for the ammonia to react with the soda.
Rub the surface with a damp rag to remove the grey marks. This is a better method for the sink, because even a mild abrasive might scratch porcelain.