Dispite our recent warm weather, which has dried up the biofilm on the concrete sheds and therefore all but eradicated the radula marks, I have triumphed and found some traces which are in the pictures below and the area they cover is roughly marked out to help you pick them out.
A radula kind of moves like a band sander, round and round like a treadmill, and scrapes the microscopic bacteria and algae into the mouth of the limpet for breakfast, lunch, dinner, tea and supper.
Picture using a sander up and down in front of yourself in short sections as you stand in one place and moving the sander front left to right as you do so. Once you have completed an arc of sanding, take a step forward and start again. The design you would make mimics the radula marks.