The flexible solar panels had been getting a bit cruddy looking and I heard on one of the sailing forums about restoring them, so I decided to follow them and see how well it would work: Stage 1 Thoroughly clean the panels. I know it doesn't look like it, but this is the state of the panels after cleaning! Stage 2 With fine wet and dry sandpaper (800) wet clean the panel, wash off, then with very fine (1,000) sandpaper wet clean again, wash off and dry. The spray with WD40 as a protective layer. You can see the difference. Stage 3 Repeat for the other panel. Apply a second layer of WD 40 and leave to dry.