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).
How the pros run it
01
Grip it like a pencil, not a hammer
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 at the counter
BUYER QUESTIONS ANSWERED BEFORE THE CART, NOT AFTER THE RETURN
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.
