Appelation Controlee wrote:

What's the best way to sand mouldings after a filler repair?

Use a filler that's softer than the timber, not car body filler. The
risk now is that any sanding will tend to remove the wood
preferentially, leaving the filler sticking up as an island. Using
chalk whiting as a filler helps, silica fillers are far too hard.

For big areas and gentle curves, use a contoured foam sander. This is a
lump of hard foam (some packing foams, or thick PU insulation board)
which is shaped to fit the moulding. If it's new moulding, take a scrap
piece of it, put some sawcuts across it and use it as a crude file to
shape your foam. Harden the surface with shellac or superglue if you
have a lot to do, then wrap it carefully in sandpaper. Stick the paper
down with spray glue, if needed.

For tiny curved moudlings, use J-Flex (the blue stuff) which is the
most flexible sandpaper I've found (woodturners love it). Wrap it
around fragments of dowel, waterpipe, plastic grout fingers etc. Watch
out if using it on your finger, as fingers change shape over the length
of a moulding and you get inconsistent results.

For tight corners, then I prefer to scrape it rather than sanding it. I
will usually make a new profiled scraper blade to go in my scratch
stock (a few minutes work to make). After a while you've already got
most of the profiles you need. Rifflers are also useful, especially if
it's gadrooned, not just a straight moulding.