Quick answer
Buy one 2 to 2½ inch angled sash brush with synthetic bristles. That is the cutting-in tool for walls, ceilings, and around trim. A second 1½ inch angled brush is optional for tight window sash. Do not use a wide square wall brush to cut a ceiling line.
Why angled, not square
The angle lets you rest the long side against trim or the ceiling while the tip lays a 2–3 inch band. Square brushes dump too much paint into the corner and wobble off the line.
Cutting in is the band you brush before the roller. Roll into that band while it is still wet so you do not see a picture-frame of brush marks.
Bristle type (Canada, latex walls)
If the can says latex, water-based, or acrylic, you want synthetic. That is the default at Home Depot, Canadian Tire, Cloverdale, and Benjamin Moore dealers.
- Synthetic (nylon/polyester) — latex and acrylic, which is almost every interior wall can
- Natural bristle — oil-based / alkyd only; they splay in water-based paint
- Foam — small furniture and crafts, not a 12×12 bedroom
How to tell a decent brush in the aisle
Wash latex out with soap and water before it skins. Hang or lay flat to dry. A $15 brush that lasts ten rooms beats three $4 brushes that streak on room one. Pair it with the right roller and tape vs drop cloths.
- Bristles are flagged (split tips) and spring back when you bend them
- The ferrule (metal band) is tight — no rattling
- A chisel edge, not a chopped-off square