Due to surface tension and other effects, the edges are always defective. This is called the selvedge. If you look closely at the support next to a film roll, you will see that the film is wider. The coater is also wider, but just right for the support. When you are done, you must trim the edges. I have to set up a trimmer for that.
In practice, Kodak coated 42" wide paper to get 40" of good coating. The edges were used for traction during movement through the machine and for the selvedge. The actual coating station was just over 40" wide. My 8" blade is 8.25" giving 0.25" of selvedge.