I've never airbrushed a figure before, but I've slogged away countless
hours in a paint booth at work, so I know all about masking. If you
need a hard edge, you'll probably want to mask it. You could free hand
it, and then hand paint the hard edges, but you'll likely get a slight
variation in paint consistancy and tone, and I doubt you'll be happy
with the results. You might try some of the masking products that
aircraft modelers use for camo jobs on their planes. Also, see if your
LHS has copies of Modeler's Resource, or other magazines geared towards
fantasy/sci-fi modelers. They usually have good how-to articles on
things like airbrushing figures.
I won't tell you that you shouldn't use an airbrush, but
IMO you don't need to. Most of the great fantasy/sci-fi painters use
airbrushes to fantastic effect, but then they work almost exclusively
in 1/10 scale and larger, and all that real estate yields a large
margin for error. My figures are all hand painted, in oils, with hand
painted Vellejo acrylic base coats. Even the 1/6th scale figures and
busts.
I like to seal figures in semi-gloss, or else a fairly thick layering
of dullcoat. In layers, dull gets a nice, slightyl less than semi-gloss
sheen, and smooths out the pain job..