To determine the "masking effect" with the Sony PRSA-CL1, I did one further experiment. Since this time it did not involve any paint brush it was not that hard for me.
Quick recap: The current status is that I have blackened the shorter triangular edges of the wedge with ceramic paint to block some of the excessive light. The two longer edges are yet untouched. The typical smudges or shading patterns are still visible, if not increased a bit.
The current experiment involved two strips of plastic duct tape. One strip is white. I glued that one along the long edge on the left side so that the bright outline on that edge is just covered and the reading area is still fully visible. Since the white tape was not entirely opaque I glued a strip of black tape right on top of it. I now have a black tape with a white bottom that faces the e-ink display. The idea is that the white tape reflects some of the light back on to the reader and by doing that lessen the smudges.
Look at the pictures and see the effect. I found it quite interesting. The first picture (DSC_0865_800.jpg) is without the tape. The second picture (DSC_0863_800.jpg) is with tape applied.
Sorry for the size and brightness differences in the pictures. I just moved and misplaced my tripod.
But the effect comes out pretty realistically.