I am trying to expose a 40" screen. It will not fit on my exposure unit by a long shot. I did a test run using small screens, exposing them using different light sources. On the first, I used a 400w halogen shop light, exposing a 230 mesh screen for 10 minutes. Every bit of emulsion washed off. On the next, I exposed screens in direct sunlight, one for 1.5 minutes, which did not pick up any half tones, and only washed out the center of the text/graphics. The next, I exposed for 10 minutes, which nothing washed out. That was all of the screens I had coated, so I couldn't run any more tests. I wanted to know if anyone else has exposed screens in direct sunlight, and if so, what was the best exposure time? Another printer in town has an arc welder they use to expose screens, but they said results are variable, and I would rather expose the screens myself than carry around a 40" screen across town to their shop.
The guys I know that expose in the sun do it for about 30 seconds. If you did it for 1.5 minutes and the halftones did not wash out only the centers of the image then it sounds like 1.5 minutes was over exposed.
I took a weekend course with a awesome screener named Andy MacDougal, he showed us that...It wasnt very long in the august sun... maybe 50 secs...I think it really matters what part of the world too...Arizona compared to the wet coast Canada, Ive got a big screen I want to do that too cept' How do you coat if your coater is only 15"?
I just typed out a long response and then saw that not only was your original question from 2008, but you already solved it. Here is my answer anyways in case someone else has a similar problem:
For a while while I was moving my shop I used only the sun. It was very frustrating at times and I would suggest leaning toward the side of overexposure a bit. Definitely test your times with a smaller screen and then use that same time for the big screen. One thing that helped me a lot when I was doing it was to build a wooden platform that fit underneath the frame and was about the size of the largest image you want to print including registration marks, and put foam over the whole top of the platform.
I guess its kinda like a table that you let the mesh rest on and then you can put a large piece of glass over the same area. The weight of the glass should put good pressure on the film and the mesh. If the frame is really large make sure you use a thick enough piece of glass (1/4" +) and have someone else help so it doesn't break. If you get a metal halide light later on you could use the same setup and adjust the distance and exposure time by using the 1/r^2 law and that would work just as well.
Exposing a screen in sunlight? 
