Second experiment with a milk-carton cardboard size 7"x10". This time prepared with two layers of budget white gesso. The cardboard still ate half of the gesso, guess I will need three layers or quality gesso next time. In all, the painting went a lot better after applying the liquid white.
Hardest part here was getting enough white into the stream and into the clouds. This time, thanks to the gesso I think, the scriptliner more or less did its job.
More experiments soon with these kind of cardboards.