In the old days, glass GH's were coated with oil based chalking white paint that would slowly wash/fade off as the season progressed and presumably, would have been thinned with turpentine. Or, they made their own, from slaked lime mixed with an assortment of additives, depending on how permanent and/or opaque of a coating they wanted.

I haven't a clue as to how you would reproduce that effect with modern latex acrylic, other than it being soluable in water, or if chalking white paint is even available in acrylc, which is nortoriously difficult to get to adhere to glass, so an oil base really would be the better choice.

However, Charlie's Greenhouse carries a specialty product made just for the purpose that could be far better and worth the cost, which isn't all that high anyway.

It's water thinable and according to Charlie's, dries opaque white and stays that way in dry sun, but becomes semi-transparent in the rain, when you need the extra light, then becomes opaque again when the sun comes back and it dries. I don't have a glass GH so have never used whitewash on it but if this stuff performs as advertised, the old timer's would have loved it.

Check it out at:

http://www.charleysgreenhouse.com/index.cfm?page=_productdetails&productid=2035&s=whitewash&cid1=-99&cid2=-99&cid3=-99