I'm currently sanding down door frames and skirting boards prior to painting. In some places especially on the door frames the previous coats of paint have been chipped by being hit by items when movig them between rooms. What I've found to be very useful in flattening down these very small areas and to feather the old paint edges is a nail file emery board.
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No connection with the company - its just where they were purchased 6 months ago. I purchased the straight boards @ 100 grit. The boards are stiff enough that they stay relatively flat even when applying pressure.