At last I found something that really works: 75% acetic acid.
I have tried alcohol, vinegar, sodium benzoate etc before but it didn't work. The picture shows where I brushed the mould a week ago with just a few drops of the acid.
(The reason was of course that some nasty mould was growing on that spot)
PS You should read on the bottle first. Acetic acid is not toxic, but quite corrosive and irritating.