That is exactly what I do for aircraft and armor where the camo can get real fine in spots.
You can paint fine lines, but that takes lots of practice and patience.
I've been foolin' around with various airbrushes for 10 years and I
feel I am only beginning to understand these tools. I have only started
doing free hand camo on aircraft.
When creating your masks, you can use regular paper and something like
blue tack, silly putty or similar to tack the paper in place. and leave
a very small gap between the paper and the surface of the model - this
will produce a soft edge. If you want a hard edge, then use masking
tape. Always spray lite coats, perpendicular to the masked areas.
3d modelling is an option
a true mental excercise in frusrtation