How to Mop Up With a Watercolor Brush

If you're painting at an angle or vertically, rather than flat, you can end up with a puddle of paint at the bottom. Don't panic, it's simply gravity pulling the paint down faster than the paper is absorbing it.

The way to deal with this unwanted paint is to mop it up with a relatively dry brush. When you place an absorbent brush onto the wet paint, it instantly soaks it up.

The brush I'm using in the photo is a Raphaël 804 Kaerell mop size 2, which has synthetic hairs that come to a sharp point.
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Place the very tip of the brush into the very bottom of the wet paint, then move it sideways to absorb the unwanted paint. This is the tricky bit because you don't want to brush more paint onto the paper nor pull off so much paint you're left with a hole. Practice and patience are the key.

The drier the brush, the better this works. So gently dab the brush dry on paper towel or a cloth regularly.

Don't stop to think with the brush still touching the paint, as it'll keep absorbing paint while you think, leaving a white hole. Lift the brush off the surface, then pause for thought.

Be careful to keep to the lower edge where the excess paint is, not to move above this, otherwise you're likely to lift off paint where you had wanted it to be, creating a hole.