With radar detectors being so common, police always have the radar turned off until you are right up close to them. Then they turn it on and theres nothing you can do. Obviously if theres no signal being sent, the radar detector isn't going to go off. I was pretty frustrated to learn this after I bought my radar detector.

I've seen two different reviews on TV of those products. They tried the sprays, a plastic thing that you put over the plate, and a few other things like that.
None of them worked what so ever. You could clearly see the plate in every trial. The spray was cheap, so I guess there isn't much to lose. But I wouldn't waste my time. There was really no benefit to any of the products.