So here is how I did it:
Take out the glass and give it a good clean if it as dusty as mine (blushes).
Take a black plastic bin bag and cut along one of the long sides, tape it with masking tape around the light with some newspaper at the top to protect the ceiling.
Spray back and forth with the spray paint to avoid drips, build up the layers gradually. Move the plastic around toe do he whole of the lamp.
Much better I think.