Welcome to the bunnings Workshop community @nettyb. It's fabulous to have you join us and many thanks for your question.
From the feedback I have received, it appears the White Knight Squirts range of spraypaints adhere best to plastics. It's a paint and primer in one so no separate primer coat is required. I would advise that you scratch or rough up the surface with some 240grit sandpaper. This will key the surface and allow something for the paint to bite into.
As with painting any plastic or PVC, it is best to do a test section to see how well the paint adheres before committing to the whole job.
Please let me know if you need further information or had questions.