I was just in my greenhouse. The day is mostly cloudy but it must be 120 degrees in there!
I have a kitchen exhaust fan in one wall, a door and high vent in the opposite wall. Everything is open altho the fan isn't on. There is even about a 10mph breeze. It's cookin'!
I don't use the greenhouse during the summer, LA47. Years ago, I worked in commercial greenhouses. We grew roses and, altho there aren't many flower holidays during the summer, the plants had to be kept at a reasonable temperature. Big greenhouses like that are essentially swamp coolers. Large exhaust fans at one end of the house, pull air through pads saturated with water at the other end. I don't suppose you are interested in installing a cooling system, are you?
I've only thought about it. There isn't much use I could gain from my 120sqft sunshed
but I can imagine wanting to use it during the summer months if I lived a few hundred feet higher in elevation.
Fifty years ago, there were still greenhouses that had whitewashed glass through part of the year. I remember asking how that was dealt with when autumn rolled around. An old guy told me that mostly, the whitewash just "wears off" from the weather. He said floor brooms could clean the glass if it was still cloudy.
I don't suppose that they used the old pioneer whitewash made from limestone and milk but, maybe ... Whitewash isn't very complicated. Don't paint the greenhouse without having some idea of getting it off but it used to be a common practice