Remove black stripes in the toilet bowl
The toilet bowl is made of porcelain and resistant to scratches. What you see in the picture (# 2) are black stripes that can be removed.
When removing stubborn lime and urine stone, I finally scraped the urine stone away with a tablespoon, which worked relatively well. Unfortunately, the spoon left black stripes that I could not get away. Toothpaste or citric acid did not help either.
While experimenting, I came up with the ideal solution for me: Putzstein and Eraser sponge this combination helped. Buy now 
Zielinsky Universal Stone, Green Stone, White Stone, Pebble, Cleaning Stone - Household Box, 1? 25,00 ?
Application:
Strongly pick up the plaster with the eraser sponge, rub off the black strips vigorously until all strips have completely disappeared after repeated use.
If you look at the photo of my toilet bowl, you can still see stripes at the top under the seat. These stripes have escaped me and noticed only through the photography.
