Not entirely sure what it is called in your preferred language folks, here we call it "fedtstift", roughly translated to grease-pen

Not always easy to find - those of you that have been in a flightschool may recall using one to mark the plastic-covered chart? Got mine at an arts- and crafts shop.

Great idea, my friend @airbum!

Generally they seem to need sharpening ever so often to be effective @bikequeen.