A major modern greenhouse operation I saw recently dealt with this by spraying the exterior with some sort of chalk solution (I think). Just made the glass pane

A major modern greenhouse operation I saw recently dealt with this by spraying the exterior with some sort of chalk solution (I think). Just made the glass panels look slightly opaque. Then they'd wash it off when they needed more light through. In this case, there was generally less rain when they wanted the solar protection, so they didn't have to worry about it washing off out of schedule too often.