Dear All, One year Cathy sent me some paint pens to use. I was marking small metal sturdy plant markers that were going in the ground. On one side I used the paint pen and on the other a grease pencil. The paint pen didn't even last a whole year. The grease pencil lasted a bit longer, maybe until the next year and that was all. So I redid them using the label machine and so far several years later they are fine. Maybe the paint pen would work better on rock, but I don't think I'd try it again. I was really glad I had used two methods. Mary Sue