Today I sprayed a couple of thin layers of the matte transparant acrylic varnish on the three remaining panels for the lighting panel. That was quite a bummer. After spraypainting them I noticed some bubbles on one of the panels like the paint had started to boil. The area became gradually larger and also the two other panels started to get these bubbles. I first rinsed the panels under the tap to remove as much paint as possible and, after drying the panels, removed the vinyl and sanded the remaining paint from the polystyrene.
Now I will have to find an alternative. The first panel I made now looks good. Maybe I will have to apply a thin layer of polyurethane paint first. This paint dries very badly (it took almost a week to be completely dry) and the result was not pretty (some places glossy, others matte). However after applying a coat of the acrylic paint it does have the wanted result. First I will have to cut some more vinyl with my cutting router.