Rendering / Repairing window Sills

[ryan kelly]

I live in a Victorian terrace house and over time the window sills have deteriorated and become uneven and rough on the surface.

What do I use to render them and get them back to a nice smooth finish again. Should I use a sand cement mix (if so what ratio) or is there a ready mixed option i can get hold of.

What preparation work should I do to the sill before re-rendering.

many thanks