Watercolour brushes used for traditional watercolour paints are best if they contain some - or all -natural fibres, as they hold the water better. Sable brushes are the best possible tools as they also hold a beautiful point. They feel soft, but are probably expensive!
Do you need sable watercolour brushes for watercolour pencils? No, because entirely different techniques are used.
Yes, you can still use quality brushes for washes, and for this you will need exactly the same type of brush as for watercolour paints. The process of placing the pigment on the paper is the same, it is just that the pigment came from shavings of pencil dissolved in water. The result should be indistinguishable from a wash created from watercolour out of a tube or pan.
However, for me, the main reason for using watercolour pencils for painting is the softening of dry pigment which has been applied to the paper from a pencil point. This needs a brush which is fairly firm but which comes to a good point. For this, an inexpensive nylon type brush is perfectly adequate and I rarely pay more than £2.50 for mine, often a lot less.
The job of your brush is to moisten, move, blend and lift. These all have a 'scrubbing' effect with the brush tip and therefore a quality brush would soon be ruined. Once your brush starts to lose the point, throw it away and replace it.
In the photograph, the two sable watercolour brushes on the right were ruined before I discovered it was best to use nylon brushes. They should have the lovely points shown on the middle three. The three on the left are the ones I am more likely to go to now.
You need a layer of dry pigment built upon the paper surface. There may be a single hue or a combination of layers of soluble colour. The method works best with several colours being blended and manipulated on the paper. This means that you do not need as many colours as you would with wax pencils. You will mix and produce variants that do not appear even in a box of 150 wax pencils! You can work successfully with 24 or 36 colours only.
I explain this process on the page about drawing clouds and skies.
Make sure that the brush is JUST wet. Avoid transferring too much water. Your brush should feel only damp when you wipe it across the back of your hand. This will aply enough water to the pigment to dissolve it and lock it into the paper surface.
Many watercolour pencil artists part dry their brush on a pice of rag or kitchen paper to avoid the pigment becoming too wet.
When you moisten the dry pigment with water your colour will intensify as it merges with the paper.
Using a pushing motion it is quite possible to move your soft pigment around on the paper.
I always suggest that if you have different levels of tone in your dry colour, that you work from the lighter tone to the darker. This keeps lighter edges light, as the brush tends to snowplough pigment in front of it. The colour mixture will darken and concentrate as it moves ahead of the brush tip on the paper.
One exception here would be if you were - for example - shading colour in a flower petal from a laid down coloured edge of the petal into the white paper of the centre (or moving colour out from a dark centre). In this case, I would pull the pigment out rather than push it with the brush. This will bring a successively paler wash out onto the dry paper.
When you have a number of colours or shades laid down dry on the paper, it is easy to work them with a moist brush to manipulate the coloured surface, blending and shaping the way the colour lies. For this, a firm brush is essential.
Because you will probably have much more pigment on the paper than you would with traditional watercolour painting, the pigment doesn't all lock down into the paper. It often sits up like an acrylic layer. The difference from acrylic though, is that your watercolour pencil pigment can be re-worked and lifted (apart from Inktense). Again you will need a firm watercolour brush for this - and probably one with a good point to it. Make sure that you have some kitchen paper to hand before you start, to be able to remove the surplus lifted colour.
Below I have enlarge part of this town scene so that you can see the detail of the cobbled street.
The underpainting, in watercolour pencil, uses a range of colours - mainly blues and purples - to give a good base for the brown added later.
You can see the dry colour on the left hand side, where it has not been touched by the brush, has a granular appearance.
However, you can also see where the damp brush has been used to moisten the pigment and move it into a cobble shape in the right hand side of the image. This has resulted in each cobble stone having light at the bottom (where the wet pigment is thin) and darker colour at the top (where the pigment has been snowploughed up by the springy nylon watercolour brush).
What you do have to watch out for is the jump in vibrancy when you add water to dry pigment. Some pencils are noted for the high strength of colour (Derwent Inktense for one). Inktense are designed to give strong vibrant and permanent colour once they have been wet.
The lower cost student colours can also give shocks as they often use chemical based pigments which are permanent.
The example shown here is of a Robin worked in Staedtler Karat Aquarelles - a very good student quality brand and good value for money.
Note the upper image is of the dry colour. The lower one shows the colour moistened with a damp brush. The pigment fills the dips in the paper grain and becomes much stronger.