The varnish thing won't work, as the new cameras all use at least 4 different light spectrums to trap. So with the older flash cameras...maybe...
Also remember that the other cameras uses video, and then a photo-print of the frame gets saved showing your number plate, so again...the varnish won't work.
Just remember that ANY way of tampering with your numberplate, or close to the numberplate, like putting up reflective tape around it, is conciddered as an attempt to obstruct the ends of justice and can get you arrested.