A typical paint job involves the following steps:

1) Wash hull
2) Sand topsides
3) Fill dings and putty flaws in seams
4) Sand topsides
5) put on a coat of primer
6) Spot fill as needed
7) Sand topsides
8) Put on second coat of primer
9) Sand topsides
10) Put on first topcoat
11) Fill flaws that show up, spot sand and prime with topcoat
12) Sand topsides
13) Put on second coat of topcoat

After several years of this process you may get to the point where a quick sanding and a small amount of spot filling/fairing is all you need to do to get a good result. However, you are likely looking at 8-10 coats of paint before you get to that point.

There may be more fairing and filling required if the hull was a bit beat up. That I am familiar with the process may be because I own a 79 year old wood boat.