You might want to read this: http://www.nelsonhobby.com/paint.html

Haven't tried it yet myself, but it's interesting where it says "CLEAR DOPE NOT NEEDED ON FABRIC.....Clear doping of polyester fabrics commonly used on larger model aircraft need not be filled or primed with clear dope. The paint is applied directly onto the raw fabric. A lighter weight finish is the result of using this paint. Crosslinker required on open fabric surfaces." I am thinking of trying this on my current project, a Sig Liberty Sport.

EDIT: Of course, they are referring to their paint product only, not others.