[IWBUMS] Painting existing walls doesn't remove dirt

[bpdlr]

When you make your own walls, and then plaster and paint them, they are spotless expanses of pure colour.

 

When you paint the walls in an existing structure, it changes the colour but the walls are still dirty. 

 

(the 4 sections from the right up to the Exit sign have just been painted white):

 

 

Here's an example of a painted player-made wall, for reference: