You will find out that photoresist films come in a couple of different thicknesses, usually 3 mil or 5 mil, on average. The thickness choice is governed by the substrate and/or depth you want to blast. If you want to blast on tile or stone, you want to select the thicker film.
You cannot blast a line deeper than it is wide, which also affects stencil production in that if you have a fine line that is thinner than the film is thick, the line quality may not be equally good in all spots. In other words, the line may not materialize.
Images that blast fine in a 3 mil film may not give you good results in a 5 mil film. The problem is that images usually do not have the same line quality overall. Any image should be examined closely; and when you detect line qualities that are too fine for a 5 mil film you need to create a copy of the image on the computer and increase the line width so it blasts well.
—Ruth Dobbins, Professional Glass Consultant