Welcome to the Bunnings Workshop community @JD1. It's fantastic to have you join us and many thanks for your question.
You'll find the Squirts range of spray paints have the best qualities for adhering to both plastic, metal and previously painted surfaces. You might like to scuff the desktop up with 120 grit sandpaper before spraying your colour onto it. Once you colour has dried you can then follow it up with a couple of coats of White Knight Squirts 310g Spray Paint Gloss Clear. This will ensure your desktop doesn't readily scratch from wear and tear. You don't need to do the clear coat on the other surfaces as they won't receive as much wear.
Please let me know if you need further information or had any questions.