Answer: I have found a recipe for greenhouse whitewash that uses several ingredients, among them, skim milk and hydrated lime (also known as quicklime--a caustic material). Here's another that is simpler and safer for your skin, eyes, lungs, etc:
1 part white latex paint to 10 parts water provides heavy shading; or 1 part white latex paint to 15 to 20 parts water gives moderate or standard shading. The latter is safe for glazing other than glass, but the former may not be. If you'd like a recipe for whitewash using quicklime, you can find it at this link:
Oklahoma Cooperative Extension Service Bulletin: The Hobby Greenhouse
Hope this helps!
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