Machine Fairing: The Variable Speed buffer/Polisher.

Fairing, can be a first step in painting depending on the quality of the surface to start.  A smooth boat with failing gel coat, normally is primed from the start with only chips and dings puttied. 

Sometimes a boat is painted due to repairs, and in the process of making repairs a side or bow is faired in to blend the surfaces together so you can't see where the original surface stops and the repaired area begins.

Most of the time, the heavy lifting in fairing is done by Machine Sanders.  My favorite, is the Makita 9227c.  It is a variable speed buffer/polisher that can be equipped with 8 inch vinyl backing pads for sand paper.

This can involve some finesse, but the gist is a variable speed sander lets you turn coarse grit paper over a large surface and remove material slowly.  Given you keep both your hands and feet in motion, and don't pause you can remove material evenly over a large area.

Applying putty, normally requires putting on more than is required and sanding down to a smooth shape.  The larger the sander, the smoother the final surface, at least on convex surfaces.  Inside corners and concave surfaces require small enough pads to define the shape without cutting grooves. 

I only sand in vertical overlapping passes with power sanders, as long board sanding uses your shoulders and elbows and easily shows vertical low spots.  Horizontal low spots are harder to detect with long boards, so if you never power sand side to side, you have less work ahead of you when it comes time to test with a long board.