What is the relationship between line quality and photoresist thickness?

Photoresist films come in a couple of different thicknesses, usually 3 mil to 5 mil, on average. The thickness choice is governed by the substrate and/or depth you want to blast to. If you are going to sandcarve tile or stone, you want to select the thicker film. At this point, you have to learn about a law of physics that says 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 good in all spots; in other words, it may not materialize.

Images that blast well with a 3-mil film may not give you good results in a 5-mil film. That is usually understood by beginners. The problem is that images usually do not have the same line quality overall. To be able to pick up on that takes a bit of experience. This means 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 line width so it blasts well. 

-Ruth Dobbins, Professional Glass Consultants