When a new student arrives at my courses, they usually have a pencil with them, at least those who have painted before, those who have not painted do not have a pencil. Pretty much everyone. who carries a pencil, has a soft “artist’s pencil”, a pen used for art-drawing, sketching or life drawing. But for watercolor you need something else, watercolor is a water technique, water and the greasy graphite do not thrive together, a lot of graphite on the paper repels the watercolor.
Pencils consist of graphite and clay mixed, a lot of graphite means that the pen is soft and black, but more clay gives a harder and lighter pen. This hardness is graded on a scale where “B” means soft and “H” stands for hard, the more B and H that are stated the softer or harder the pen is. The softest pen has the designation 9B and the hardest 9H, in the middle are HB and F. In the USA another scale is common (Conté / Thoreau scale), a numbering from 1 to 4 where No. 1 corresponds to 1B and 4 is the same as 2H, nr2½ is “normal hard” the same as F in the European scale.
As I have mentioned before, water and graphite are not friends, watercolor flows off a graphite line without getting stuck on the paper. For that reason, when drawing on watercolor paper, you do not want to leave too much graphite on the paper. I use a regular HB pencil when I draw on watercolor paper, it is soft enough to give a clear line but not so soft that it disturbs the watercolor. Alternatively, you can use a slightly harder pencil, F or 1-2 H but not harder, then you probably scrape the watercolor paper, it’s like drawing with a nail.
If you use a regular pencil, wood-enclosed graphite pen, you must always keep this pencil pointed, it is not a good idea to use a blunt pencil for watercolor, if you want to avoid constantly sharpening the pencil, use a thin mechanical pencil.
I myself always use a 0.5 mm mechanical pencil in hardness HB. It has just the right softness and is always sharp.