Every day, every hour, every emulsion & coating thickness is different. Even in California.
Read:
Exposure using the sun!??!
Learn to measure UV energy for your stencil with a US$10 21 step scale. You can measure the sun with two sacrifice exposures using a Stouffer 21 Step Scale to measure the stencil hardness, then shoot your serious stencils.
Exposure FAQ Screen Making Products how to measure exposure Measuring UV Exposure
It is the primary job of the screen maker to learn how to judge exposure. Invisible UV energy reacts with emulsion sensitizer and hardens the stencil so it won't dissolve with water and rinse down the drain. You block UV energy with a positive during exposure so the image area dissolves when you develop the stencil with water.
Screen makers can
only expose one side of the stencil, so it is important for the UV energy to
move through the stencil, all the way to the back, where the squeegee blade will rub the stencil where it is weakest.
When the stencil doesn't hold in the mesh, it didn't get enough exposure. Start by checking the suggested exposure time on the product
Technical Data Sheet. These
estimated time charts are based on known
commercial screen printing UV light sources on 305 mesh. .
How long to expose using the sun? 
