Hugh,
That was a good article and timely. Recently I took pictures of a couple cars to sell on the Internet. I covered the license plates with duct tape for the photos for OPSEC. I pulled the tape off after I was done, and two days later I noticed a very nice dirt film obscuring the letters exactly where the tape was. So just a 3 cent piece of tape placed on different letters or numbers and then removed will collect road film and make your plate unreadable and easy to explain or rub off if you’re pulled over. – B.C.