I use these pens to write on a blackboard film. At the first use it is necessary to pump quite a long, then it can be written very nicely. The peak is relatively fixed, so I have the impression that the bar a little thinner than the indicative 4 mm; However, I have the pins only recently in use, maybe that will change even more. The color is very strong and bright and can be removed by damp wiping residue. Am very satisfied so far.