Most of those sprays work by putting small reflecting crystals all over the plate, so if there is a flash from the camera, it wipes out the plate. They also make reflective or refracting licence plate covers, but again, see #3.
1) They are temporary, and wash off after time.
2) They work only at cameras with a flash (ie, only at night), and often they don\'t work all that well.
3) In many jurisdictions, anything you do to obfuscate your licence plate is illegal. So trying to (unsuccessfully) get out of that 80 dollar light ticket could cost you demerits and a much larger fine if you get pulled over and they notice it.
This is what it\'s \'supposed\' to do.
![](https://psx5central.com/community/proxy.php?request=http%3A%2F%2Fwww.olavsplates.com%2Ffoto%2Fusa-fl_tip91m.jpg&hash=73f6706dd6a3d8b84b89d66e36604e2fb0a1966e)
If it actually worked for once, and they wanted to hunt you down, here\'s the same plate with a bit of photoshopping:
![](https://psx5central.com/community/proxy.php?request=http%3A%2F%2Fwww.olavsplates.com%2Ffoto%2Fusa-fl_tip91m_close.jpg&hash=d645559a6d919ed2da91279dcb02c8ee1d13c26c)
Clearly visible as TIP 91M... and they gotcha.