Brushes are sized by width. As a rule of thumb, bristles should be about 50% longer than the brushes' width (and even longer on very narrow brushes).
Brushes with natural bristles should not be used with water-based paints. Before being used for the first time, natural bristle brushes should be soaked in linseed oil (not resting on their ends) for a couple of days in order to swell the bristles inside the ferrule, which helps to prevent their pulling out. Synthetic bristles, good for water-based latex and acrylic paints, can be destroyed by certain lacquers, varnishes, and other paints containing strong solvents.
There are four main classes of brushes:
Made in widths from 3″ to 6″, in ½ inch steps from ½ to 3 inches, and from 3 inches to 5 inches in steps of a whole inch. The 5″ and 6″ sizes have been almost killed by the rise of the paint roller. It hardly matters for do-it-yourselfers. The wider the brush the greater the demand placed on rarely-used muscles in the hand: the weekend painter is generally more comfortable with a 4-inch brush, and women often prefer 3″. Wall brushes are used for walls, ceilings, siding, and floors. They have a square edge.
A 2″ synthetic bristle, chisel edge, trim brush
These brushes are made in three styles: round (½ to 2 inches in diameter); oval, and flat (1½ to 3 inches). They often have a chisel edge. A 1-inch chisel edge brush would be a good choice for trim; a 2-inch chisel edge could be used for doors, shelving, and window frames.
Short videos by a manufacturer of high-quality brushes illustrate the use of round and flat trim brushes.
Flat 2″ to 3″ brushes. Be sure the composition of the bristles is compatible with the varnish. A brush used for clear varnishes should never be used for paint, since it is impossible to clean all the paint pigment out of a brush. By Murphy's Law, some pigment will come out into the next varnish coat.
On the left, a 6″ black China bristle brush. On the right, bristles of Tampico fiber. Both are made by Wooster.
Very wide brushes, 5″ or 6″. The brush on the left is about $60; the one on the right is $13 (2020 prices). The difference arises from the two very different types of coatings that are applied to masonry and stucco.
One is sealant, which is typically extremely thick; the other, paint. The brushes used to apply sealant have bristles of Tampico fiber, or very heavy, stiff, synthetic bristles.
Paint is usually applied with a roller with a long nap. The brush on the left is made for those parts of a job where a brush is needed.
Because the surface is so abrasive, for painting masonry a cheap brush may be a good idea, unlike almost any other class of paint brush.
Good advice on painting household walls: how-to-paint-a-wall-like-a-pro
Another, with less emphasis on prep and more on use and care of tools, some of them proprietary.
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Last revised: 1 August 2020.