Hello, as a deruster or rust converter I got a finished mixture with 85% phoshoric acid. Unfortunately the manufacturer cannot give me any information about the application. It is about a) rusty steel brake discs made of unalloyed or low alloyed St 07 b) rusty grey cast parts from GG20 or GG25 The phoshoric acid solution is quite thin and the parts have sometimes poorly accessible places. Therefore the brushing method is not possible. Is it possible that you have some experience of how long you can leave such parts in the bathroom? Greetings of the car parker