If your solar lights aren't illuminating as brightly as they used to, clouded plastic over the solar cell is the most likely culprit. Wipe the plastic off with a damp cloth to see if the plastic is cloudy or just dirty. A coat of lacquer or nail polish helps make the cloudy plastic clear again.
Clean the Plastic First
If the plastic covering the solar cell looks white or cloudy instead of clear, a regular cleaning lets you see if the cloudiness is simply dirt and pollution buildup that can be wiped away, or if the plastic looks the same, even after cleaning.
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Wipe loose dust and debris away with a dry, soft cloth.
Dip a cloth or sponge in slightly soapy water, wring out most of the moisture and wipe the plastic down again.
Rinse the cloth or sponge and wipe the plastic again. Allow the plastic to air-dry, or pat it dry with another cloth. If the plastic still looks cloudy after it dries, it requires additional treatment.
Revamping Cloudy Plastic
Fix that cloudy plastic covering the solar cells by coating it with a thin layer of clear nail polish or a spray-on clear lacquer. These clear coatings render the plastic clear once again. Cover the areas surrounding the plastic -- either the housing for the cells or the lid of a light with a built-in solar panel -- with painter's tape before applying the chosen clear-coat material. Peel the tape away once the coating dries.