Greenhouse managers use a number of techniques to cool a greenhouse in the spring through summer and one of them is the application of glasshouse paint or whitewash to the windows.  These compounds are rolled or sprayed on and are available commercially.  Typically, they wear off by the time protection becomes unnecessary or else they are removed by spraying with water along with a gentle brushing.

The control of heat and light in a greenhouse is quite a complicated balancing act.  For more information on the subject please take a look at this Fact Sheet from the University of Massachusetts.

Courtesy of NYBG Plant Information