I use Parafilm M for masking before spray painting. Once you stretch the stuff, it will adhere well to smooth plastic — yet it has no adhesive. (Lab workers use it to seal vials, beakers, dishes, or test tubes, similar to stretching cellophane over a bowl or plate.) After stretching it uniformly, I let it rest for a minute. Then I cut off the unstretched ends. 

Instead of placing the Parafilm M on the model part and slicing it with a hobby knife, I press it onto a cleaned plastic cookie bag (I like Chips Ahoy) and gently trace the pattern I want using a gel pen. Then I cut out the mask with a scissors, peel it off the plastic, place it on the model, and gently lay my finger on the mask and let the warmth make the mask adhere. I find I have more control with a scissors than a blade, and I don’t risk having the blade pull and distort the mask. So far the results have exceeded my expectations as a nervous new modeler.

Tip submitted by:
Marin YouchahClifton Park, New York