OD - I'm gonna ask a kinda dumb question here - are you planning to repaint the control surfaces with Ultracote paint? Or, cover the painted control surfaces with Ultracote film?

If you're going to cover the darkened painted surface with white Ultracote, apply a bit of film on a control surface asis. Do a perimeter-only patch to see if the darker surface "bleeds" through the white. Reason being is that the lighter colored Ultracote films are a bit transparent - maybe 10 - 20%. Not much but it can be noticeable especially when overlapping a darker bit of film (e.g. white over blue). Also, use a low heat for this test as Ultracote adhesive will hold with low heat (but not aggressively). If you turn up the heat the adhesive will get real tenacious and you will have a hard time removing the covering from a painted surface. Much harder than using the same heat on clear, unprep'ed balsa.

HTH