cutting along window frame

frames you have to cut along the window frame with a design knife before painting. May I know why this must be done? Thanks in advance.>>

I haven't seen the video, but I think I know why. When you cut along the frame it makes a groove in the plastic. This groove acts as a barrier that the paint has a hard time flowing past, or into, and onto the next section. The situation is similar to painting model cars, or aircraft with deep panel lines. If you just spray the whole thing at once, sometimes the paint will not flow into panel lines, or door/tunk seams, and will leave light lines. So you spray a light coat on those seams, then spray the rest. On an aircraft canopy you can use this "problem" as a preventive cure to keep the paint from bleeding over onto sections you need to keep clear. You still have to mask of course, but the groove acts as an additional barrier.

When once you have tasted flight, you will forever walk the earth with your eyes turned skyward, for there you have been, and there you will always long to return. --Leonardo Da Vinci

I forgot to mention this, sorry. The tutorial on that site, there are 2 issues being talked about on that topic. One was painting with a fine tipped brush and an airbrush. The question I posted was regarding painting with a fine tipped. The tutorial on using the airbrush was a little different since it covered masking then cutting along the car's window frame to expose it then airbrushing it..

topic. One was painting with a fine tipped brush and an airbrush. The question I posted was regarding painting with a fine tipped brush.>>

Oh, ok. But, given the physics of liquids, I'm betting the point either tutorial is trying to illustrate is that the creation of a groove will act as a barrier to paint flow onto adjacent surfaces. Hell, I could be wrong, it happened once before, long ago, in a galaxy far, far away. ; )~

When once you have tasted flight, you will forever walk the earth with your eyes turned skyward, for there you have been, and there you will always long to return. --Leonardo Da Vinci

I guess they didn't mention this. How deep should the cut be?

Chad