I have a user grade J3 Slim that I want to repaint the numbers on. I just tried nail polish and a fine tip brush, but I'm not too happy with the results, so I am gonna try these and see if it is any better!
Do what I do paint the numbers then get a bit of A4 paper and wipe over the top, that will remove the paint around the indent and leave the paint in the indent then let it dry. If any paint is still left it is thin so can be removed with paint thinner, but only wipe it over once as it will soften the paint in the indent.