You need to pick the right exposure time for the product you are using, no matter if you are using a washout film or dry-process film; those guidelines are there for a reason. I strongly suggest not relying on the timer on the Letralite as it often turns out to be inaccurate. It is best to use a digital timer, which greatly increases your chances of producing good exposures time after time.
With the washout films, you do not have a lot of leeway, maybe 10 seconds in either direction. The dry-process film needs at least 90 seconds to get any satisfactory image; anything under this time will not give you good results. This film allows you to go higher in exposure time by even a few minutes before you may encounter overexposure signs.
The exposure light diminishes in its output of light over time and will eventually affect your exposure. When you notice the quality of your stencils is deteriorating, you only have two choices: either up your exposure time or change the lightbulb. Sometimes, the light will not turn on immediately after you let go of the timer dial, which also affects your exposure. There are several details you have to closely pay attention to.
Another important factor: the age of your film. If you are not using your film within a year’s time, it may not process properly anymore.
—Ruth Dobbins, Your Professional Glass Consultant