If the paint is semi matt or shiney it will be paint or if its matt may well be the stone chip protectorJust purchased a mk1 mx5 and planning to spruce it up a bit over the next few weeks.
Underneath the doors the black paint (stone chip protection?) is really tatty and needs a good seeing to.
What sort of paint is used, looks satin to me?
You could use aerosol satin black to cover it up firstly scruff up the sills with a brillo pad or red scotchbrite