Thanks DM. I've since sprayed a can of transparent blue, which has reminded my how bad at spray painting I am. Unfortunately I had the nozzle oriented incorrectly on the first coat, so I've got some banding going on. I also had quite a few runs, but they have mostly sanded out. This wasn't the colour I had in mind - it's rather more green than I had expected. I shall get another can of colour, and hopefully it will darken up and even out.