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Hi All,
One of my students here wants to know if it's possible to reduce the line width of brush strokes already painted.
I can't imagine that it is but it's always worth asking!
Pete

FX > Erode.

To accompany slowtiger's answer, here is an example using the Erode effect (under Stylize in the FX stack panel) Changing the height setting too much might change the look of the brush you are using.