Glossy paint? to protect from the Sun? or Flat paint?

Consider plywood.

why not just up solar screen and that will let you see out and the sun will be blocked from coming in.... or you can do what business that go under do and use white wash on the inside of the glass and just rub it off when you want to use the windows again.....

Durability of exterior paint primarily depends on prep. Scrape, TSP wash and prime any bare wood, then use a high quality exterior latex enamel, and it is unlikely to peel.

White paint lasts longer because you can tint it with titanium oxide, an excellent ultraviolet screen, and white doesn't get so hot. Gloss paint will not get dirty as fast and will clean up with a hose more easily than flat paint. Most houses need to be washed once a year to keep them from looking shabby.