Quote Peter Steyn: "I use 'optivision' it's the material used for the adverts on car windows etc. (Those where you can't see inside the vehicle but the driver can see out) it is made of a series of small holes. Just place a piece over the blade b4 blasting then remove and place another piece in a different position, the more you move it about the more interesting the emerging pattern....hope this explains! My local 'Sign Shop' is more than willing to keep off-cuts for me"
