>>10834848Standard gundam markers can also bleed into softer plastic over time making any lines you make fuzzy. This even happens under topcoat should you make the mistake of putting it over the pen.
I have also had them cause one of the expensive magazine tie in add on kits bandai used to release shatter, although I do think that was a freak case. Fortunately it was a part that wasn't used on the Zeta Honoo I was building.
I always just recommend some cheap acrylic paint for panel lines. No risk of it acting up with the plastic or the topcoat and it can be used if you do a full paint job or not.