i recent tried to use a razor point pen on panel lines, but i did it before i painted in an attempt to "pre-shade".
i don't know if the ink is the wrong sort, or if the whole idea (using a fine tip pen) was hare-brained, but when i attempted to paint over it, the paint 'activated' the ink (or perhaps it does not dry in 24 hours) and the ink "bloomed" thru coat after coat of paint.
i thought i would have to use 2 bottles of flat white to cover a 1/48 Wildcat!!
i am not sure why this failed so dismally, and the method you describe works so well.
suppose it's the type of ink?
the paint & thinner (airbrushed Tamiya Acrylic) ?
it will be a while before i go down this road again!!
ed.