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Hallways are usually very difficult to touch up, depends on the lighting but patches usually show up more at oblique angles. If you have primed the patches with Gardz you'll have a better chance of the filler not 'grinning'.
With the sash brush you can trim the end square with scissors. When you're painting sashes you normally trim the end to a point. It becomes like a stencil brush with a square end.
Cheers, I'll have a look at sash brushes. Unfortunately it's the hallway at the bottom of the stairs so it's a big, awkward wall.