After skimping on brushes, the second most common mistake is using the wrong type of brush. The best brush for today's water-based paints is one made with synthetic bristles--nylon or polyester. Natural China bristle brushes, made with hog, ox, or badger hair (they're called China bristle because that's where most of the hair comes from) should only be used with oil-based paints. Use them with water-based latex and the natural fibers will absorb water, swell up and quickly lose their shape.
Of the synthetic bristles, nylon bristles are soft while polyester are generally stiff. Softer bristles offer a smoother application, while stiffer bristles offer better control, so the best synthetic bristle brushes for all-around use are usually made with nylon/poly blends and have a medium flex. If you're painting with a high-build product like Aura, which is a little thicker than conventional paints, choose a brush with an extra-firm flex. This will give you excellent control, even in hot or humid conditions. For thinner-bodied semi-gloss or high-gloss trim paints, many pros use 100% polyester brushes with chemically tipped filaments, which help eliminate brush marks and leave behind an extra-smooth finish.
There's a number of other subtle features to look for when choosing a brush. A good indicator of quality is flagging, or the split ends that you can see at the tips of the bristles. Synthetic bristles need flagging to leave behind a smooth finish so avoid brushes that don't have it. Compare bristle density too; paint is held in place by the space between the bristles, so the more bristles a brush has, the more paint it will hold. Quite simply, you'll spend less time loading the brush, and more time actually painting with a good brush. Good brushes are tapered too, while cheaper brushes have bristles that are all the same length. Tapering the bristles so that the brush is thicker toward the ferrule--the metal ring that holds the bristles against the handle--gives the brush more stiffness and you more control over the application.
Handle shape and balance is a matter of personal preference, though most users like a beaver-tail rather than a straight profile. What you're looking for is a brush that feels comfortable in your hand and that's easy to control. Wider 3- and 4-inch brushes are fine for walls and other broad, flat surfaces, but for trimming around a window sash you'll probably want a 1 1/2-inch to 2 1/2-inch brush. In fact, 2 1/2-inch angled sash brushes are the most popular brush because they're comfortable to use and can trim around most areas.
Finally, give the brush a shedding test--slap the brush against the heel of your hands and then give a few bristles a tug. If some pull out of the ferrule easily, look for another; there's nothing more aggravating than a brush that leaves bristles behind in the paint film. You may not pick out the perfect brush immediately, and you certainly won't find one in those bargain-priced packs, but, remember, this is a tool you'll be working with closely that controls the final look of your job. Taking a little extra time and spending a few extra pennies will repay in a far superior finish.
