Lead covered window sills

[norfolk house rob]

I have finally started preparing my windows for painting.
Plenty of rubbing down, dust, fixing and filling , but they are looking in pretty good shape for oldies.
My problem is the exterior window sills, they are lead covered, no great problem in its self, but they have been painted first with some sort of black stuff and then with what i would imagine to be gloss.
My question is a: what is the black stuff likely to be,
b: do i only remove looses flaky paint as this leaves a very uneven finish, and what should i repaint with as a primer and top coat.
If any one has any ideas I would be very grateful.
If I dont respond until this evening please forgive me as I am making the most of these dry spells.

 

[NT]

Some car paints are designed to work on lead undercoat, which used to be standard on cars. Some more modern paint types are incompatible with this, and will fall off. Unfortunately I forget whch type is which but you can google.

NT