Wall and ceiling cut-in. A room's cut line wants a 1-1/2"–2-1/2" angular sash brush (Hyde Millenium 47370-series, Richard B5 sash family); even at 7/8" a trim brush is narrow and slow for that run.
Oil, alkyd, and varnish. SRT is for waterborne coatings; natural white or black bristle lays oil paint, varnish, and oil stain better (Hyde Extra 47380-series, Richard bristle brushes).
Flat fields. A flush door field, a wide flat baseboard face, or a shutter panel is a flat brush's or small roller's job; a 7/8" trim brush is for edges and members, not fields.
Fine muntins and slivers. For divided-light bars and slivers a 7/8" overshoots, drop to the 5/8" (13415) or 3/4" (13418).
Dip a third to half of the filament and never to the ferrule — paint dried in the heel splays a brush permanently. Tap off the excess rather than wiping. Tapping each face inside the can keeps the reservoir the large-capacity SRT is meant to hold; wiping the rim strips it. Set the chisel edge and pull one pass. Present the thin tapered edge to the sightline, start just short of the end, come back into the corner, and draw the line in one smooth stroke as the tips gutter paint to the edge — on a wider brush the single-pass discipline matters more, since a doubled stroke on a broad face shows. Keep the wet edge in fast waterborne paint, working sections short enough to join wet to wet. Rescue the brush at the heel: rinse waterborne paint under warm water, clear the base of the fill first, comb the filaments parallel, spin or shake dry, and store in the keeper to hold the taper.
Asked before buying
BUYER QUESTIONS, ANSWERED BEFORE YOU ORDER
What is SRT?
Solid Round Tapered synthetic filament — large capacity, smooth lay-off, built for fast-drying waterborne paint.
Why 7/8"?
It fills wider casing and panel-molding faces in a pass while still cutting a controlled line — the wide-trim width.
13421 or 13521?
Same size and job; Elegance is the SRT finish upgrade, the 13521 the standard tier. Buy up when the trim shows.
Oil or varnish?
Use natural bristle for those; SRT is for waterborne paint.
Making it last?
Keep paint out of the heel, clean the heel first, comb parallel, store in the keeper.
