UV mapping , when colouring the outer corners of the canvas , other faces are affected which are not unwrapped O.K here goes
I've got a simple plane , which is subdivided in 64 equal parts (8*8)
I select the inner 16 faces and unwrap them , in the object data tab I give it a name , lets's say "this one"

Now , in the (cycles) material editor  I create a mix shader ,which consists of a simple diffuse shader and a texture map that will have the image for the selected inner faces )
The texture map get's its coordinates from the UV map node( set to 'this one '') .
Now , when going into the image editor /view mode we can see our selected /unwrapped inner faces and start drawing .
I make sure that the unwrapped  uv,s are centered in the image editor , surrounded by enough empty space 
As expected , only the inner faces are affected and thus coloured  

Here's the thing , as soon as I draw in one of the corners of the image editor (just a tiny 1 pixel dot is enough)  ,all  the other faces are also affected which are not unwrapped at all .
How come ?
So my question is ,are the corners of the image editor some kind of boundary ?
If you unwrap any part of a mesh, a uv map is created. In a UV map all vertices in a mesh needs to have a UV coordinate. Blender gives not yet unwrapped vertices the coordinate U=0.0, V=0.0 (bottom left corner).

Since all your corner faces share the same uv coordinate they will get effected in the same way.

You could use the Face Selection Masking for Painting and give the corner UVs enough space on the UV map to not get effected by the unwrapped faces. Another way would to give the corner faces a different material.