I use a sharpie black permanent marker. I paint this on all sides and edges. If you get too much on it can be how 2 tap a cell phone removed with fingernail polish remover or acetone. The correct amount of blackening is when the adjustment pot is approximately at mid setting when the circuit is tilted so about half of the cell is illuminated by sunlight and half covered by the light blocker. Repeat this painting of the cells until all 4 cds cells are adjusted.