Mythbusters proved that the only thing that would work is going over something like 160 miles an hour (Might have been even higher) or building a contraption that flips the tag over or the like. They tried the spray stuff two or three times and it never accomplished anything, under any conditions.

PS: That stuff will make your plate super-easy for cops to read from a distance at night thanks to your tag lights.

PPS: State law also says you can't put anything that obscures or discolors the liscense plate in any way shape or form (So those sunglasses colored bent plate covers for example). You CAN use clear flat plastic covers for keeping your plate clean.