Learned the hard way.
Never use Deleter colour pens on Peter Pauper Press sketchbooks. Sure, the results look cool. But the ink goes right through the page. (Certain other brands of brush pen marker, too, while I'm at it.)
Prismacolor Premier fine line markers, though? That's another, happier story.
I still like the sketchbooks. I just need thicker paper for some types of pen.