Blending acrylic paints (1), filbert brushes - "Kookaburra mugshot" WIP1

Ever found you can't get the blended effects you're after with acrylic paints? Perhaps you're using the wrong brushes!

I was inspired a few days ago to start a new acrylic painting to test out my new set of Atelier interactive acrylic paints. I took the kookaburra reference photo (see Fig. 1) a couple of weeks ago at one of my base-camping photography/birdwatching weekends at Mebbin National Park (NE NSW, Australia). The kookaburra was a bit of a character - very determined to encourage us to feed him at our campsite by posing attractively for photos on nearby branches!

The photo itself was taken with my 100-400mm Canon telephoto lens, so had a beautiful bokeh blurred background of green. I tried to emulate this with the acrylics, but wasn't getting the effect I wanted (see the background green from the Work in Progress 1 photo; WIP 1). I couldn't cover enough of the canvas-board with a round brush (as I'd usually use for watercolours), couldn't get sufficient paint movement on the board with a mop (for watercolour washes), and I kept getting sharp lines wherever I placed the tip of a flat brush (you can see these in WIP 1).

I've since realized that the brushes needed for acrylic work can be very different to those for watercolours, and that much of my problem was caused by the unforgiving shape of the square flat brush I was using. Instead I should have been blending acrylics with a 'filbert' brush.

Filbert brushes: I've included a photo of some filbert brushes of different brands, with different sizes, overall shapes, bristle types and characteristics in Fig. 2. Basically, filbert brushes are similar to flat brushes, except that the 'toe' (or tip of the bristles) is rounded at the edges to form a half-oval rather than a straight line at the end. The filbert brush was actually named because it represents the shape of a hazelnut (a species of hazelnut is called a filbert). Used side on, the filbert brush can produce a thin line, but used flat it can produce a broad stroke with a rounded end.

From experiments using filbert brushes, I've worked out a few things about how to use them:

Having a variety of sizes of filbert brushes is quite useful, because with different sizes it is possible to create different degrees of blending or gradation between colours (for example, small brushes naturally produce shorter regions of blending, and larger produce longer gradations)
Different lengths of bristles behave differently - this has to do with the natural spring in the bristles, and how you're likely to use the brush. It's a personal thing, but I find there's a compromise between bristle length and softness when choosing the perfect brush for blending acrylics, so try out a few brands. It's also worthwhile noting if you're ordering online that the 'sizes' quoted by different brands are often not equivalent between brands.
Softer natural fibres such as sable seem to work better (although the brushes are considerably dearer) because their hairs stick together better when the brush is wet.
If you're using a synthetic bristle brush and getting 'streaky' type effects, you can remove these by ensuring that your brush is fully loaded with paint or medium - do this by loading a small amount of paint, and then brushing the flat of the brush across a test surface (back and forth, both flat sides of the brush) multiple times, until the streaks disappear.
It often helps to work with both flat sides of the brush (rather than always stroking in one direction with the one side of the bristles) because it is easier to get an even blend.
Old filbert brushes can be used for rougher applications such as dry-brush techniques.