Thank you for all of your suggestions, I'll keep them in my mind : )
I'm trying out new paper, I have this old East German song book called "Ringsum erwachen Lieder". It's from year 1956, so the songs are pretty interesting... I got it last spring from our schools attic, it looked really worn out and it had suffered some water damage too, but I took it anyway. The paper really isn't that nice to paint on, it's quite porous.
Before anyone asks, I don't know any German and I don't know how to read notes. The page was selected randomly and the song (which by the way is called "Beherzigung") isn't related to the painting any way.