The product we are using to fair with calls for primer. The primer has some build up and helps by turning the surface all one color. When you add fairing compound the low spots are red and the high spots are gray after pulling across the surface with a long straight edge. Gives you a guide to sanding. The board sander should be used to sand to the primer but not beyond. After the second pull you should sand back to the same level as you did the first time.
You should follow the directions from the manufacture of the fairing compound as to surface prep. If I was mixing my own with epoxy I would not prime but use the same process when fairing.
I am not sure what Quick Fair calles for but am sure it is in the directions.
