I have used a sharpie twice for preshading.
One time with enamel paint and it worked fine. I had no problems what so ever.
the other time with acrillic, this worked ok, but Future over the acrillic paint lightly attacked the sharpie underneath.
Although this is not a big big problem (there where just a few runs) I am not sure if I would use a sharpie again with acrillic paint and Future.