Stencil thickness influences depth of cure, just as ink thickness affects drying or curing time. Moisture in the emulsion coating For optimum exposure, the emulsion must be completely dry. Residual moisture in the coating will inhibit the crosslinking process, and you will effectively get an underexposed stencil, regardless of the length of exposure time. The open area and fabric thickness, which are products of the mesh count and fabric thickness, are major determinants of the amount of emulsion deposited on the mesh. White mesh causes halation, or light scatter, which shortens exposure times at the cost of resolution.
Dyed meshes minimize light scatter and provide better resolution at shorter exposure times.
All fabrics will cause some degree of halation, which may or may not be a problem depending on the nature of the artwork. Lith films have a clear background and an opaque image area, which is what you want from a positive. At the other extreme, vellum positives have a translucent background and an image density that will allow light to pass through to the emulsion. Subscribe online to Screen Printing, the only magazine covering the entire field of screen printing.