  We reexamine the functional renormalization-group theory of wetting
transitions. As a starting point of the analysis we apply an exact equation
describing renormalization group flow of the generating functional for
irreducible vertex functions. We show how the standard nonlinear
renormalization group theory of wetting transitions can be recovered by a very
simple truncation of the exact flow equation. The derivation makes all the
involved approximations transparent and demonstrates the applicability of the
approach in any spatial dimension d\geq 2. Exploiting the non-uniqueness of the
renormalization-group cutoff scheme, we find however, that the capillary
parameter omega is a scheme-dependent quantity below d=3. For d=3 the parameter
omega is perfectly robust against scheme variation.
