Safest Cleaning Options
You don't have to buy expensive cleaning solutions for your greenhouse's polycarbonate walls, or panels. The safest way to clean the part of them on the inside of the greenhouse is to use a mild soap, such as a dish-washing liquid mixed with water. Just a few drops of the liquid dish soap mixed in 1 gallon of water is sufficient -- enough soap to create some suds at the water's top but not enough soap to discolor the water. Spraying water from a garden hose onto the side of the polycarbonate panels that is on the outside of the greenhouse should remove the dirt and dust. Don't use the soap-water mixture on the outside of the greenhouse.
Other Options
When a stronger solution is necessary to clean the polycarbonate panels, read the greenhouse manufacturer's instructions to see what is safe to use on the panels. In many cases, window cleaner with ammonia as an active ingredient cleans polycarbonate panels without damaging them. Naptha-based cleaners usually work as well. Don't use any ingredient specifically designated as harmful by the greenhouse's manufacturer. Rinse the panels with clean water after using any chemical, even soap, on them.
Harmful Products
Several chemicals commonly found in household cleaners can damage polycarbonate panels during only one use. Avoid using substances such as bleach, pine-scented cleaners and other abrasive cleaners, for example. Most all-purpose household cleaners include at least one potentially damaging ingredient. Also, stick with soft, cotton cloths when cleaning, avoiding brushes and squeegees, which can scratch the finish on the panels.
Proper Procedure
In order to clean the polycarbonate panels properly and safely, dip a clean, soft, cotton cloth into a bucket of the mild soap and water mixture, and gently rub the greenhouse's interior walls with the cloth. An effective method is to start at the top of each wall panel and work your way downward, dipping the cloth in the soap-water mixture often to keep it wet. Rinsing the panels with plain water removes soap residue. Use only plain water to clean the outside of the greenhouse. Dry the panels inside and/or outside the greenhouse with a clean cotton towel, if desired, to prevent water spots. Always start from the top and work your way down when you wash, rinse or dry, inside and outside the greenhouse. When using a cleaning solution, complete all steps on one panel at a time to keep soap residue from drying and water spots from forming.