Finally got round to getting some chip-resistant paint on the outer sills yesterday.
Over the last few weeks, I had put the pale brownish look to the paint finish as just mud fling from our rural roads, but mud it wasn't . . . it was a beigy colour showing through the black paint on the sills!
Had half a tin of Teknox stone-chip paint left over from a previous job, and decided to give the sills a wipe over with detergent/water to remove any actual mud, and then a wipe with alcohol. Was amazed to find the remains of the black paint started to come off with water


Not sure who had painted these sills, and what with, but a few months of car wash had obviously begun to remove it!
Got most of the rest off, and then noticed what was underneath was good Sable body colour, with a stone-chip coating underneath.
Wonder whether that stone-chip was original factory finish in black, and a subsequent respray (parts of the car have been) put body colour over it? All pics of non-sporty BXs I found seem to have black sills.
Anyway, a couple of coats of the solvent-based Teknox have been applied, so looks a whole lot better. Before painting, I straightened out one or two short sections of the vertical sill seam which had been used (as they do) as a jacking point. A squeeze with a vise supported by a trolley jack got the nice straight line back again.