As brushes go, there are all manner of shapes, materials and qualities, so finding what's great for you can be a trial and error game. Let's see if we can find an easier way to play!
First, what medium are you working with? Will you be using the brushes to paint, apply glue or brush metal leaf off a surface?
Watercolour Brushes tend to be soft and are available in a variety of natural hairs and synthetic bristles. The most prized watercolour brushes are made with "sable" hair, Kolinsky being one of the most popular. These are made from the tail hairs of the male Siberian weasel's winter coat. Because the weasel does not do well in captivity they are trapped and killed for their tail hairs. I'll let you decide where you stand on that one. Other natural hairs include squirrel and camel hair, neither of which maintain a fine point for detail and they are generally considered to be children's brushes. There are some wonderful synthetic watercolour brushes on the market, though, that replicate the qualities of sable brushes, are easier to maintain and are much more budget conscious.
Oil Brushes are usually stiffer and often made of Boar bristles (hair). The stiffness of the brush allows the artist greater control over the thicker oil paint and also leaves the brush strokes that we associate with oil painting. Because they are cleaned with solvents, instead of water, and used with oils the natural bristle tends to maintain it's spring and doesn't degrade as quickly as brushes that are constantly used with water, but you can never get a real sharp point on a round or create a fine line with a flat on it's edge like you can with the synthetic brush. Synthetic oil brushes are usually stiffer than synthetic watercolour brushes.
Acrylic Brushes are generally synthetic because natural hair brushes don't stand up to the water and mediums as well, breaking down faster. Synthetic brushes also allow the paint to slide off the brush onto the canvas or paper, where natural hair brushes tend to clog with quickly drying acrylics. When working in acrylics you can use synthetic watercolour brushes (soft and short handled) or synthetic oil brushes (stiffer and long handled) and of course there is a range of stiff nesses in between the two.
Multi-purpose brush uses that include blending pastels and brushing metal leaf off a painting surface are generally stiffer than watercolour brushes and natural or synthetic fibres both work well.
Shapes of Brushes
Flat: This brush is flat and wide with a square tip. It is generally used for blocking in and applying colour generously as it holds a good load of paint. You can paint bold strokes with the flat side and finer lines with the edge.
Bright: A bright brush is basically a flat with shorter bristles.
Round: As it’s name implies, a round is round. These are generally used for details and painting lines.
Fan: The fan brush is flat. It is used for smoothing, blending and creating things like trees, shrubs, grass, fur and other special effects.
Filberts are a member of the flat brush family but instead of a square tip they have an oval one. This makes them useful for blending as well as daubing in foliage and creating soft edges on clouds. The side can also be used to create thinner lines. These are often a favourite of landscape painters.
Rigger/Signature: These are fine, long bristled brushes that, because of the length, hold a good load of paint. They are used for fine lines and writing signatures. They get their name because they were traditionally used to paint rigging in maritime paintings.
House Painters brush: A good quality painters brush can be a big help when painting large canvasses. It’s a larger version of the artist’s flat or angle brush. These brushes also hold a lot of paint so be prepared to use lots of your colours!
Angle brushes: These brushes are handy for getting into corners, much like the house painters cutting brushes. You can also use them as you would a flat or bright.
Comb brushes: The bristles at the tip of these brushes are thinned out compared to the lower portion of the bristles. This allows the brush to be loaded with sufficient paint but to apply it more sparsely. Use the comb to create grass or fur effects.
Dagger brushes are soft and flexible and shaped like a dagger... one edge straight ad the other edge curved. They are wonderful for creating lines of varying thickness, as in lettering or decorative work. You should use fairly thin paint to take advantage of this unique brush.
Deerfoot brushes: These brushes began their lives as rounds and then got chopped off at the end. They are terrific brushes for creating leaves, shrubs and anything else that has a repetitive round or oval pattern. Use by holding the brush fairly upright.
Shapers: Paint shapers look like brushes but are made of rubber/silicone. They come in a variety of shapes and sizes and allow the artist to remove or shift paint to create effects that cannot be achieved with regular paint brushes.
Stencil brushes: Anyone who is familiar with stencilling will recognize the stippled effect that these brushes provide. The brushes are loaded with paint, tamped on a paper towel or rag to create a “drybrush” situation and then applied at a 90' angle to the painting surface. The resulting mottled effect can create the appearance of plaster, soften edges and assist in the blending process.
Tooth brushes: Spattering allows the artist to create snow scenes, add texture to rock faces, develop backgrounds and blend colours in a painting. Old toothbrushes are perfect for this, creating spatter by using the thumb, or by rubbing the brush, ladened with paint, over a piece of wire mesh.
Sponges are great for creating the suggestion of bushes and trees, rock faces and anything with texture. Sea sponges work best because of their naturally random patterns. When using sponges it is always a good idea to change the direction of imprinting to help maintain that natural effect. To build a natural layered effect start with the darkest colour and gradually work to the lightest colour.
Brush handles are always handy to create texture, lines and designs in your wet paint. Alternately you might use a notched mortar trowel, or anything else hard to create textures and patterns.
Fingers: Get right into your work of art by manipulating it with your fingers. Fingers work well as a blending tool, an eraser, a paint applier, and when you use your whole hand, a schmoosher!
Natural Materials: Think outside the art store! Use pine needles, a cedar branch, flowers and seed heads, feathers, sticks, moss, leaves… whatever you can find that makes interesting marks.
Old Brushes: Never throw out an old ruined brush, because even those have their greatness for creating textures, special effects and things you would never use your good brushes for!
Today's Painting Tip:
When painting with acrylics use a liquid/polymer medium to thin your paint, not water! If you thin your paint with too much water you can break down the bonding process of the polymer binder and when your paint is dry the pigment will easily brush off your painting surface. The medium is made from the same polymer binder as your paint, so you can thin your paint as much as you want and the polymers integrity will still be maintained. I like to thin my polymer medium with 25% water. This way I still maintain the integrity of the medium and the paint but it handles much more like water. By the way, you can thin any acrylic product with up to 25% water with no problems, otherwise, water is only for washing brushes!
Update on the Painting of Jane's Photograph:
In today's photo I have taken the painting beyond the flat colours of the last stage by adding lighter colours to the rocks. I've worked from Burnt Umber, to Burnt Sienna to Burnt Sienna mixed with some Cadmium Yellow and then used Cadmium Orange to highlight the shapes of the rocks and show the sunlight on the edges. I have also refined the sky. The texture that was applied with the gel medium is now starting to be enhanced with the layers of paint. Getting close to being finished... I'm getting excited!