A few years ago, we picked up these great color changing solar light orbs. After years outside (including our harsh winters), the solar cells had clouded over and the lights had mostly stopped working. I tried wiping them down with a wet rag, but the cloudiness was not just dirt. So I thought I'd try one of the fixes I'd seen online. On Pinterest I have seen folks use clear nail polish and acrylic sealer. I figured the acrylic sealer would be a better long term fix, so I propped my lights up, wiped off any dirt, then sprayed them with clear sealer.
The results were downright miraculous. The solar cell went from cloudy white to clear and black again in seconds. I let the sealer dry, and they were still perfect black. You could once again see the metal lines in the solar cell.
Unfortunately, I can only give this As Seen on Pinterest review a "Sort of Works" verdict. After they dried, I put them back outside, and even after several sunny days, the orbs were still working about as well as before (one was dim, and the other two I have were not working at all before or after the spray). It could just mean that my lights still have something else wrong with them (like needing to replace the battery that holds the charge or some loose wires, corrosion, etc...). So if you have some clouded over solar lights, it's worth giving this fix a shot to see if it solves your problem. It definitely works at clearing up the solar cell, but it's not a cure-all for old solar lights.