My experience:
1. use a better quality marker instead of the Sharpie if you may find one
2. paint the cards one by one
3. paint on the thin and long plain of the cut edges
I don't know how many people understand the "cutting angle" I mentioned, but it is very important. By finding the cutting angle, you may paint the edges effeciently (with only one stroke) and prevent the edges from being ruined. Everyone may paint his cards well and make them look awesome, however, I guess most of them can not faro successfully after the edges are painted.