I painted a very dilute high chroma, cool acrylic pre-wash across the top part of the painting and misted the wet area with rubbing alcohol to create a distant, faint organic texture seemingly beyond the top window panes.

Note the puddling of wet glaze. A usual reaction of wet medium on Yupo.  More about that another time. It is not worrisome on this painting.

Also note a piece of blue painter's tape on a corner of the chair-back to preserve that small portion that will remain bright white.

To view the alcohol-acrylic reaction double-click on the photo.

You can try it yourself. Paint a thin acrylic glaze on a small square of Yupo. When the glaze has dried in only a few 1-2 inch areas, spritz the entire wash with 1-2 squeezes of a small spray bottle containing 99% rubbing alcohol. Watch the reaction between the acrylic medium and the alcohol. A very usefull technique to know.