Q&A for How to Expose Photo Emulsion for Silk Screen Printing

You should always expose it for four minutes. But be careful when exposing using sunlight because you might burn it.

The purpose of the emulsion is to block ink, preventing it from going through the mesh. Photosensitive emulsion hardens when exposed to light, becoming insoluble in water. By blocking light where the image will be, that emulsion remains soluble and can be removed allowing ink through the mesh.

This is partly a factor dependent on the specific emulsion used, so YMMV. Some emulsions require UV light to harden, which is why you can find tutorials using halogen bulbs and sunlight to expose the emulsion. For emulsions that harden without requiring UV, you can use a wide variety of light sources like fluorescent tubes and LED arrays. Your lower limit on light intensity (lumens) is a function of how long you want to wait for exposure to harden the emulsion. The upper end is bound by two factors, burning the emulsion, which will mostly be a wattage problem, and bleed through exposing covered areas (lumens).

20 hours would be needed, as the sun isn't enough alone. If you are using a fan with your hand, that could just be 5 hours.