The first try I used tap water. The liquid turned bad in a day, so I could only do one coat on the paper. Second try (this one) is made with alcohol. It doesn't turn bad and it paints better. It also doesn't make the paper wrinkle.
I took a peek at the contact printed paper and it made quite a good print. I'm going to give it one more day, tonight I'm going to unveil contact and camera sheets.
@Cliveh: the camera has a 80mm diameter lens without aperture. I focussed it on a big dark tree with a light background, facing the sun for as much contrast as possible. I don't expect much from it, but that's what the experiment is for. Also, I can't see if it works, without taking the paper out of the camera. When I take it out, I can't replace it on the exact location. So taking it out means end of exposure. I expect to see some light streaks from the sun passing by. We'll see tonight.

#edited# Using gasoline for an experiment? Just for fun? Do you know gasoline costs 1,83 a liter over here?