As you proceed to model your objects a common fault often creeps in – more than one vertex on top of each other. This happens in various situations but most commonly when an extrude or scale operation is abandoned. Vertices on top of each other are invisible and yet they can play havoc with model shading. This is such a common fault that creeps into your blender drawing with such annoying regularity, that it’s best to get into the habit of periodically following this routine:
- Select the object,
- Toggle to edit mode,
- Switch to wireframe view,
- Go into vertex select mode,
- and box-select the whole object.
- Then perform a Merge Vertices –> By distance operation .
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A panel will pop up asking for the Merge Distance:
You need to decide how close the vertices need to be from each other and enter this distance.
Then press 
and look at the bottom of the viewport which briefly flashes up telling you how many vertices have been merged:
You can change the Merge Distance in the parameter panel as often as you like and see the number of vertices that Blender wants to remove. Only when you click 
in the viewport are the vertices actually removed.