Sand using, er... sandpaper, then prime and spray!
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Halfrauds sell a project pack of various grades of wet & dry sandpaper for about £2.50, IIRC.
If you end up having to buy paint (£6), primer (£6) and lacquer (£?) then it's cheaper to buy it. I had all the necessary materials apart from the wet & dry, so I thought I'd give it a go.