A paint brush has one job: to transfer paint from a container to a surface in a smooth, even film. The quality of the brush determines how well it does this job. The difference between a cheap brush and a good one is not subtle -- it is visible in every square foot of finished surface.
Natural bristle brushes -- China hog bristle, ox hair, badger hair -- work because of a physical property called flagging. Each natural bristle splits at its tip into dozens of finer sub-tips, like a hair splitting at its end. These flags hold paint in the micro-spaces between them, increasing the paint-holding capacity of the brush and releasing it more gradually and evenly as the brush is drawn across the surface. A brush with more flags holds more paint and leaves fewer tracks.
Synthetic bristles do not flag naturally. Manufacturers attempt to replicate the effect by mechanically splitting the synthetic fiber tips, but the geometry is less consistent and the paint-holding capacity is lower. For water-based latex paints, synthetic bristles are the correct choice -- natural bristle absorbs water from the paint, swells, and loses its stiffness. For oil-based paints, varnishes, alkyd enamels, and shellac, natural bristle is always better.
The ferrule is the metal band that secures the bristles to the handle. On a quality brush, the ferrule is stainless steel and the bristles are set in a plug of vulcanized rubber -- epoxy is also used, but less durable. A brush with a loose ferrule will shed bristles into your work. The first thing to check when buying a brush is whether the ferrule is tight and the bristles fan evenly rather than clumping.
Brush care determines whether a good brush lasts for a decade or gets thrown away after the third use. For oil-based paints: clean with mineral spirits, working the solvent through the bristles from ferrule to tip repeatedly until the solvent runs clear. Then wash with dish soap and warm water, reshape the bristles, and store flat or hanging by the handle. For water-based paints: clean with warm water and soap immediately after use. Paint that dries in the ferrule cannot be removed -- it separates the bristle bundles and ruins the brush.
Wooster Brush in Wooster, Ohio and Purdy in Portland, Oregon are the two American brush manufacturers with continuous production histories dating to the 19th and early 20th centuries respectively. Both still make brushes in the United States. The brushes that professional painters specify -- and that painters who have been in the trade for thirty years reach for first -- come from these companies, because the bristle selection and the ferrule construction are consistent in ways that cheaper brushes are not.
A fifty-dollar brush is a thirty-year investment if it is cared for. The per-use cost is lower than a two-dollar throwaway brush that leaves bristles in the trim paint and requires three coats to achieve what a quality brush does in two. The economics of the good brush are simple. The limiting factor is usually that the person buying the brush does not plan to be painting for thirty years. But then the brush outlasts the plan.