Additionally, a fan blowing on a screen that is freshly coated with direct emulsion acts as a dust shotgun, embedding thousands of particles of dust in the emulsion layer. Each of these dust particles can become a pinhole. After dirty mesh, the two most likely causes of pinholes are dirt on the glass of the exposure unit and underexposed screens. You must clean the glass regularly. If the exposure unit is of the flip-top type, the side of the glass away from the screen should be washed at least once a week.
Underexposed screens are another common cause of pinholes.
Some screen printers deliberately underexpose screens to hold fine details. Others inadvertently underexpose screens because they have not correctly calibrated their screen-exposure procedures. All the major emulsion manufacturers sell exposure calculators, which come with comprehensive instructions for use. Rather than deliberately underexpose your screens to hold fine details, use a different emulsion, a higher mesh count, dyed mesh, capillary film, film positives instead of vellum, a better light source, or all of these items. Image Developing Contrary to what many screen printers believe, the image-developing process is not often a cause of pinholes.