My old Rover uses those push on screw caps of the type that grip in the slots of a Pozidriv head - and some are missing. Now they're readily available in the sheds, but not in the grey colour I need.
Dunno what they're made of, but ordinary oil paint doesn't bond as it does to drainpipes.
I've tried Halfords vinyl paint and that falls off too. Any suggestions? Preferably in aerosol form.