Apply a dollop of acrylic to your palette and grab a flat brush. Don’t scoop the paint like a spoon! Instead, press one side of the bristles into the paint. Turn the brush over, and gently drag the opposite side through the paint. Do two or three strokes on the palette before moving on to your painting. You want a brush that is loaded all the way through the bristles, but isn’t overloaded with paint at the tip.
Why? The stroke starts changing once the paint begins leaving the bristles. An overloaded brush might lay down a heavy ridge of paint at the beginning. An underloaded brush might start with a dark, saturated mark, and end up with a dry, scratched-out trail. There’s nothing inherently bad about either of these results, but they can be annoying if you’re trying to paint a clean shape, or an even edge. A well-loaded brush should give you enough paint for the entire stroke, and allow you to control the stroke with pressure and direction.
Paint consistency plays a role here. A thicker acrylic may require a slightly damp brush for it to glide smoothly, but too much water can reduce opacity, causing the loaded paint to run toward the tip. Wet your brush before loading if necessary, and blot off any excess water with a paper towel or cloth. Now load it with paint. If it immediately runs or spreads into a puddle on the palette, your brush may have more water than you need for an opaque stroke.
Make some test strokes on acrylic paper or a canvas board. Try three different levels of brush load. Start by barely touching your brush to the paint, then do one stroke. Load both sides of your brush with paint, and do a second stroke. Then load the brush heavily, and do a third stroke. Keep your brush pressure relatively constant throughout. The light load will likely turn scratchy or transparent partway through the stroke. The medium load should remain more consistent. The heavy load may have ridges of paint along the beginning or sides of the stroke. Let the strokes dry before you compare them. Wet acrylic can mask the difference between loads.
The shape of your brush will affect the appearance of the stroke. A flat brush holds paint across its full width, which can be good for painting solid color blocks and clean straight lines. A round brush concentrates paint at the tip, which is good for painting smaller shapes, but can easily become overloaded. A filbert has a broad body and a rounded edge. Rather than applying exactly the same amount of paint to each brush, observe where the paint sits in the bristles and how it comes off the brush.
Pressure can help adjust the load. But pressing harder with an underloaded brush won’t give you more paint. It will just push the bristles apart, exposing more of the surface beneath. Excessive pressure on an overloaded brush will squeeze out a thick line of paint around the stroke. Start with moderate pressure, and adjust pressure as desired to widen or narrow the stroke, soften or sharpen edges, etc.
Before making an important stroke, inspect your brush from the side. The bristles should be coated in paint and able to flex freely, but shouldn’t be clumped with a big ball of paint at the tip, nor should the tips be near-dry. If you aren’t sure, try making a test stroke on a scrap piece of paper. If the brush is delivering an even stroke without a thick ridge at the beginning or a scratched-out end, your load is likely in the right ballpark.