I normally use a fine point, but occasionally find that a medium point is the answer, especially when it comes to strong lines. The ink flows a lot easier in a medium biro, but one hasn’t got the control needed for delicate work that a fine point biro has.
Fine points are excellent for cross-hatching and for pointillism (adding dots).
Smooth paper of at least 110 gsm is required. The thicker the paper, the easier it is to apply the biro. I find that 240 gsm or better, smooth watercolour paper is ideal.