I've heard of people being prosecuted for having non-rusting plates that they had made themselves. I suppose the logic is that if they allow people to make their own plates there's no control over the number that appears on them.
I had plastic plates and when I was stopped I was told they were illegal and I would be fined if they weren't replaced with transport issued ones. I put on the metal plates I got from transport and they rusted out. I was then stopped by transport and told I needed to replace the rusty ones with new ones or I would be fined. Can't win.