I have read very mixed reviews of PhotoBlocker spray. Some people say it works well enough and others say it's pants. Maybe it's because the product works better on some plates than others.
Here in Hong Kong, plates are predominantly the British plastic-type ones with a sheet of arcylic backed with letters and a reflective strip. In the USA, the product's primary market, plates are of the pressed metal type, without a thick plastic coating.
I was told by a supplier that it only works on metal plates, and not metal plates. Probably it's due to paralax and internal reflection, or something.
I just purchased a set of metal plates for HK$200. These are legal here in HK and I can temporarily switch back to the regular plates if I get a fix-it ticket.