Oil-based paint, varnish, and enamel where the finish must flow dead flat.
Synthetic filament will carry oil coatings, but the glass-smooth lay-off of an oil enamel belongs to soft natural bristle — Richard's black-bristle PREMIER (80452-class) and Hyde's white-bristle Extra (47380-class) are the same-family tools built for it.
Match the fill to the can, not to habit.
Broad open fields — a 1-1/2" brush painting a whole wall is a long afternoon; cut the edges with this and roll the middle.
Wide cut-in runs — for the long uninterrupted wall-to-ceiling line, the 2-1/2" carries more paint and covers the same line in fewer strokes (§5).
thumb and first two fingers on the ferrule, handle laid back over your hand. The pencil grip is what gives a straight cut its fine wrist control. Load a third. A cut-in brush carries paint in the belly of the bristle, not the tip; overloading floods the line and drips. Lead with the long point and let the weight of the loaded brush do the pressing — you are steering, not scrubbing. Set back, then move to the line so first contact doesn't splay a blob right on the edge. Keep a wet edge: cut a section, roll it in before the cut flashes dry, and the two melt together — this matters more with fast-drying modern latex, which is exactly what a synthetic brush is built for. Ease off the load before you lift to avoid a fat stop mark.
Asked before buying
BUYER QUESTIONS, ANSWERED BEFORE YOU ORDER
Latex or oil?
Water-based — latex, acrylic, primer, low-VOC. For oil/varnish use the natural-bristle sibling.
Why angled?
The long corner draws a straight line against an edge; the angle is the cutting geometry.
Why so narrow — is a wider one better?
Narrow is better for tight and interrupted lines (muntins, small pockets). For long open wall cuts, size up to the 2-1/2".
80801 or the soft-grip 80811?
Same brush; pick the handle your hand prefers.
How do I keep the line crisp over time?
Comb the heel and spin it dry after every use — a splayed heel is what ruins the point.
