I fitted and mdf cill to the kitchen window a couple of years ago.
This morning I noticed (behind some strategically placed clutter -
wondered why it was there!) that a small (2 cm stretch of the 'nose'
has had the paint chipped off and is a little swollen.
I'd rather not replace the whole board (could be a right pain with
trims etc) so I thought I'd see if I can repair it. My first thought
was to route a new edge, but the router would foul on the wall exactly
at the end where the problem is.

It should be pretty simple to carve back to good stuff leaving a
shallow-ish groove along the top edge of the nose, but what then? Is
there something particularly good for filling mdf? Should I prime/
paint it before building it up (though it might not give a good
surface for adhesion)? Or should I cut out a bigger section and
replace with a new piece of the nosing? (Might be tricky to get dead
right of course).

I'm sure someone has had this problem before, among all you
knowledgeable people, so any tips warmly received.

(Why do I have visions of angle grinders, SDS chisels and car body
filler?)