Painting sharp corners is bad

When painting joinery that has external corners on it like doors or windows, it is important that your painter decorator uses sandpaper to rub-off the corner - or Arris - so that the paint will maintain its thickness at these vulnerable area's.

Where the corners have not been rounded-off, the surface tension in the wet paint film tends to reduce the thickness of the paint at the corner. When the dry paint is knocked it can chip or flake off at this point.

If the corners have been rounded off, the paint will tend to cling evenly to the corner and keep a constant thickness.

The recommended radius for the corners is 3mm, but that is quite a lot. In practise any rounding off of corners will help.