Because out of the can, Behlen's is too thick to "spray" without
cutting by some percentage. Most varnishes and other finishes must be
reduced when spraying. Some folks reduce the first coat when brushing
Too thick a finish may result in "orange peel" effect when sprayed
onto a surface.
In fact, this evening I sprayed a 3rd coat of Behlen's which was
reduced only by 30%, and I noticed the orange peel effect. Tomorrow,
I'll sand lightly and resume applying thinner coats (50% reduced). Air
mixture may play a role in orange peel effect, but generally its
attributed to excessive thickness of the varnish being sprayed.