Also, the white residue might just be limescale. If so, then any limescale remover should work. Personally I've found Lidl's W5 limescale remover to work very well, though there are many other brands available, usually based on citric acid. Just apply & scrub the area with a toothbrush or similar. If it is scale, then you'll see bubbles form when it reacts with the acid. You could also try white vinegar, this being acidic, should also help.