Paint Brushes: Sizes, Bristle Types, and What to Buy for an Extension

Walk into a freshly decorated room and your eye lands on the cut-in lines: where wall meets ceiling, where gloss sits against emulsion, where two colours meet. Those lines are cut in by hand with a brush. A wobbly line ruins a room that is otherwise perfectly decorated. Most homeowners reach for a cheap pack of three brushes from the supermarket and wonder why their lines look shaky. The answer is rarely technique. It is the brush. A £2 flat brush with stiff filaments cannot hold a line. A £6 angled-sash synthetic can.

What it is and when you need one

A paint brush is a bundle of filaments bound into a ferrule and fixed to a handle, used to apply paint to surfaces where a roller cannot reach or where roller texture is unwelcome. In a typical extension decoration phase you will use brushes for three things: cutting in at the edges of walls and ceilings before rolling the large flat areas, painting skirting and architrave after the walls are finished, and painting doors and door casings.

Brush and roller work together. You cut in with a brush first, taking a 50 to 75 mm band of paint along every edge of the room (ceiling, corners, around windows, along the top of the skirting). Then you roll the wall, bringing the roller as close to the cut-in line as you can without touching it. The rolled paint blends into the wet brushed band and the join disappears.

Brushes are also the primary tool for gloss work on woodwork. You can spray skirting and architrave with an HVLP or airless sprayer for a flawless factory finish, but that requires masking the room floor to ceiling, owning or hiring a sprayer, and the experience to avoid runs. For a homeowner doing a single extension, brushing is the practical choice.

Sizes and shapes

Decorator brushes come in widths from half an inch (for fine detail) up to five or six inches (for priming large flat panels). For extension work you need a small set that covers cutting in, woodwork, and the edges of doors.

If you are buying one brush for an extension, buy a 2-inch angled-sash synthetic. The angled cut on the filaments lets you follow a wall-ceiling line or the edge of a piece of trim much more precisely than a flat-tipped brush. The 2-inch width holds enough paint for a reasonable run without constantly reloading, but it is narrow enough to fit around door frames and into internal corners.

A sensible starter set for a full extension is three brushes: a 1-inch or 1.5-inch for narrow architrave and sash, a 2-inch angled for cutting in and skirting tops, and a 3-inch flat for skirting faces and door panels. That will cover every painting job in the build.

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Bristle and filament types

The filament material is not a detail. Match it to the paint or the brush fails.

Synthetic filaments (nylon, polyester, or a nylon-polyester blend) are stiff, consistent, and hold their shape in water. Modern water-based paints (emulsion, water-based gloss, acrylic eggshell) are formulated around synthetic brushes. A good synthetic filament has a flagged tip (the end of each filament split into fine branches like a split end of hair) which holds more paint, and a chisel profile (the brush head cut at an angle) which gives a crisp cutting edge. This is the default brush for 95 percent of domestic decorating now that water-based paints dominate.

Pure bristle (also called natural bristle or hog bristle) is stiff, hollow, and naturally flagged. It absorbs oil-based gloss, oil-based undercoat, varnishes, wood stains, and shellac without swelling. Use pure bristle for oil-based gloss, oil-based undercoat, varnishes, wood stains, and shellac. Do not use it for emulsion or any water-based paint: water makes natural bristle swell and go limp, and within five minutes of use the brush is useless.

Blended filaments combine natural and synthetic. They are a compromise brush designed for both paint types but excelling at neither. If you want a single brush for everything, a blended Hamilton Perfection will do. If you are buying more than one brush, go pure synthetic for water-based work and pure bristle for any oil-based gloss.

Which paint system are you using? Most modern extension decorating uses water-based paint throughout: contract emulsion or vinyl matt on walls and ceilings, water-based eggshell or acrylic gloss on woodwork. That means synthetic brushes for the whole job. Oil-based gloss is now rare in domestic work because it yellows within two years and dries too slowly, but some homeowners still prefer it for its self-levelling finish on skirting. If you are using oil-based gloss, put a separate pure-bristle brush aside for it and do not ever put it in water.

The cutting-in technique

Cutting in is where painting stops being a matter of applying paint and starts being a skill. The mechanics are simple, but they have to be right.

Pour paint into a paint kettle. Not the tin. A kettle is a small plastic or metal bucket with a handle that holds about half a litre, and it is the only sensible way to carry paint on a ladder or around a room. Fill it no more than halfway. Paint a wet-edge strip of masking tape across the rim of the kettle if you want a clean spot to tap the brush on.

Dip the brush into the paint no more than one third of the bristle length. Any deeper and paint runs into the ferrule, down the handle, and onto your hands and the floor. The loaded section should be the tip of the chisel, not the whole brush.

Tap the brush lightly against the rim of the kettle to knock off the drips. Do not wipe it on the rim. Wiping scrapes most of the paint off the bristles and leaves you with a dry brush that will drag across the surface and leave brush marks. Tapping keeps the brush loaded.

Approach the cut-in line at a shallow angle, brush handle roughly parallel to the line. Make a first pass 10 mm away from the line to unload some paint onto the wall without the risk of straying over. Then make a second pass directly along the line. Use the chisel tip of the brush, keep even light pressure, and move slowly. The tip of the bristles rides the corner where wall meets ceiling. Your wrist stays steady. Your eye follows the line three inches ahead of the brush, not directly on the brush.

When you come to a stop, feather the wet edge back into the wall so it blends. A cut-in band of 50 to 75 mm is ideal. Any less and the roller struggles to meet the brushed paint; any more and you are brush-painting the wall, which leaves visible texture.

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If you are cutting along masking tape rather than freehand, the technique is the same but you can press a little harder. Brush toward the tape, not along it. Brushing parallel to a tape edge forces paint under the tape; brushing into the tape from the wall side does not. Remove the tape while the paint is still wet, pulling it back on itself at 45 degrees, or you will peel dried paint off with the tape.

Gloss on woodwork

Gloss, eggshell, and satinwood on skirting, architrave, and door frames need a different brushing technique to emulsion on walls. The paint film is thicker, self-levelling, and far less forgiving of brush marks because the sheen shows every stroke.

Load the brush generously. You want more paint on the brush than you would for emulsion because gloss is thicker and you need to lay it on in a single working pass rather than scrubbing it into the surface.

Apply in short crosswise strokes to get coverage onto the wood. A skirting board might take three or four crosswise strokes per 300 mm run to load the surface.

Immediately "lay off" by drawing the brush lightly in one direction only, with the grain, bristle tip barely touching the paint. The laying-off stroke pulls the brush marks in line with the grain and lets the paint self-level into a smooth film as it dries. Lay off once and move on. Do not keep brushing. Water-based gloss in particular flashes off quickly, and each extra stroke after the lay-off leaves drag marks that set permanently.

Work toward a wet edge, never away from one. If you are painting a length of skirting, start at one end and work along the full length in one go. Do not stop halfway, go for a cup of tea, and come back to continue: the paint at the stopping point will have tacked off, and the next brush pass will tear it. Plan door panels, skirting runs, and architrave so one continuous working face moves across the surface.

On a panelled door, paint the mouldings and panels first, then the horizontal rails, then the vertical stiles last. The stiles are painted top to bottom in one stroke with the grain, laying off any overlap onto the adjoining rails.

What to buy

Brushes span a wide range of quality and price. The gap between a £2 supermarket brush and a £6 decorator brush is bigger than the gap between the decorator brush and a £25 Purdy.

Multi-packs are the best value if you are buying a starter set. A Harris Seriously Good 3-pack (1 in, 1.5 in, 2 in) typically costs

at Screwfix or Wickes. A Hamilton For the Decorator 3-pack with angled sash brushes runs at B&Q. A Purdy XL 3-pack is closer to on Amazon and through decorator merchants, but the brushes will outlast the extension.

The one upgrade worth making on any tier is to the 2-inch angled sash. Cutting in is the visible work. Spend there, and save on the larger flat brushes for skirting faces where the finish is less critical.

Cleaning and storage

Every good brush can be ruined by careless cleaning. Every mediocre brush can be kept going for years with decent care.

For water-based paint, rinse the brush in lukewarm water the moment you stop painting. Work the bristles under the tap, pushing water through the ferrule and out at the tip, until the water runs clear. Spin the brush between your palms over a bucket to shed water, reshape the chisel by running a finger along the tip, and hang it to dry.

For oil-based paint, clean in white spirit or a dedicated brush cleaner. Work the bristles in a jar of white spirit until the colour bleeds out, then repeat in a second clean jar, then wash the brush in warm soapy water to remove the white spirit residue. Spin out, reshape, and hang.

Between coats on the same day, do not wash the brush at all. Wrap it tightly in cling film, pressing all the air out so the paint cannot dry. A wrapped brush stays workable for up to 24 hours at room temperature, longer in the fridge. This saves 20 minutes of washing per coat and protects the brush from repeated drying cycles that break down the filaments.

Once fully dry, slide the brush back into the original plastic sleeve (the "keeper" that came on the brush when new). The keeper holds the chisel shape and protects the filaments in a drawer. If you have thrown the keeper away, a strip of brown paper folded around the bristles and rubber-banded to the handle does the same job.

Where you'll need this

• Decoration - cutting in at corners, ceiling lines, and painting all woodwork including skirting, architrave, and door frames