Watercolour brushes were traditionally made from natural sable, the most luxurious hair available. They hold a copious amount of water and keep a good point, but they are expensive and too soft for the purpose of using watercolour pencils.
The newer synthetic brushes are just as good and a lot cheaper, meaning you won't worry about replacing them when they are past their best. Reserve the sables for traditional watercolour paints.
The base technique when working with watercolour paint is to first lay down a wash of pigment. You mix your pigments and water in a palette and use a large brush to apply the colour to the paper.
When using the tip of watercolour pencils you would normally lay down dry colour and then take a wet brush to dissolve the pigment to create a wash. If the brush holds a lot of water it can dilute the pencil pigment too much giving a wishy-washy result.
It is still important to use a brush that comes to a fine point, and most nylon types are perfectly adequate while being economical.
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 reach for now.
You need a thin layer of dry pigment laid down on the surface of the paper. This may be a single colour or a mixture of several water-soluble colours. You will need fewer watercolour pencils than you would wax, as you will mix and blend them rather than use them straight from the tip.
I explain this process in more detail on the page about drawing clouds and skies.
Make sure that you do not flood the surface of your watercolour paper with too much water. Your brush should just feel damp when you wipe it across the back of your hand. This will apply enough water to the pigment to dissolve it and lock it into the paper surface.
Many watercolour pencil artists wipe their brush onto a piece of rag or kitchen paper to prevent the pigment from becoming too wet.
When moistening the dry pigment with water your colour will intensify as it merges with the paper.
Using a stiff brush, it is possible to work large areas of paint by lifting and moving the pigment around on the paper.
If you push the brush across the paper the pigment will build up in front of the brush just like snow would in advance of a snowplough. This leaves a lighter area behind the brush as the pigment moves forward.
If you drag the brush you will eventually run out of pigment and the end of your stroke will be lighter in value.
By working these motions together, you can build up areas of colour that vary in tone.
The pigment doesn't all soak into the paper, some of it will remain laying on the surface. Taking a firm watercolour brush you can lift this extra pigment and then use kitchen paper to absorb it.
Inktense pencils resemble ink more than watercolour paint, and once the water dissipates the colour will no longer lift.
I have enlarged 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 that will be 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 on the right-hand side of the image. This has resulted in each cobblestone 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. For example, 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 European Robin shown here was worked in Staedtler Karat Aquarelles - a very good student quality brand and good value for money.
Note the upper image shows the dry colour. The lower one shows the colour moistened with a damp watercolour brush. The pigment fills the dips in the paper grain and becomes much stronger.