Can I use sunlight to burn my screens if I run a small shop with minimal equipment?
Yes, I used to do this when our exposing unit was too small for the screen, or if the bulb had burned out and we couldn’t wait for a new one.
As an aside, this is a method I generally do not recommend, but if you have no other options, it is possible. To begin, use spray adhesive on the film and then press it onto the shirt-side of the screen to be burned. Make sure that the adhesive is sprayed evenly so as not to leave spots of heavier deposits on the film. The trick to this method is not to expose the other side of the screen (squeegee-side) while it’s being handled under the sun.
To avoid exposing the other side of the screen, I’ve used a cart on wheels that is at least as big as the screen. Cut a piece of foam that is at least as thick as the screen is and fits on the inside of the screen. Cover the foam with black T-shirt material, then lay the screen with the substrate side up and place the film in position. Don’t forget to put the film face down.
Now place a piece of glass on top of the screen that is at least as big as the screen and place some weighty object, such as a quart of ink, to push the glass, film, and screen down onto the foam beneath. Be sure to put the weight around the design on the film so that it can be exposed evenly by the sun. The weight will promote the best contact between all the layers. Now simply cart the screen to where you want to expose it.
Another tricky part is getting the right emulsion exposure. You may think that on a cloudy day, the UV rays aren’t passing through, but they are, and on a sunny day, they can be even more intense. Then there is the midday overhead sun which is much stronger than the afternoon sun in regards to the amount of UV available for exposure. I strongly recommend not using a pure photopolymer emulsion as those are just too sensitive for this technique. It may take some experimentation to get the exposure just right.