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Why streaks appear in the first place

Most streaks and lap marks come down to one thing: painting over a section of wall that has already started to dry. Emulsion paint forms a skin quite quickly, especially in a warm room or on a sunny wall, and if you roll back into that half-set film you drag it about instead of blending it. The result is a visible ridge where one pass meets the next, and a patchy sheen that catches the light at an angle.

Textured finishes hide it better than silk or eggshell, but no paint is immune. The good news is that lap marks are almost always a technique problem rather than a product problem, and they are straightforward to avoid once you understand what the roller is actually doing.

Start with the right roller and the right amount of paint

Before you touch the wall, match your sleeve to the surface. A short-pile sleeve for smooth plaster, a medium-pile for lightly textured walls, and a long-pile only where the surface genuinely needs it. Using a long-pile sleeve on a smooth wall loads far too much paint and leaves a stippled, uneven finish that looks like streaking even when the coverage is fine.

• Load evenly: roll the sleeve through the paint in the tray several times, then roll off the excess on the ramp until the sleeve is damp rather than dripping.
• Keep a consistent load: reload before the roller starts to sound dry or drag. A dry roller is the single biggest cause of patchy walls.
• Don't over-dip: half-loading more often beats a heavy load that spits and runs.
• Check your tray: a shallow, ridged tray lets you distribute paint across the whole sleeve rather than just the middle.

A damp roller covers roughly a square metre before it needs reloading. If you find yourself stretching to two or three, you are already thinning the paint out.

Roll in a W pattern, then fill in

The W technique is the workhorse of a streak-free finish, and it is worth practising on a scrap of plasterboard before you commit to a wall. The idea is to spread fresh paint over a wide area quickly, so you always have wet paint to blend into.

• Start near the top corner of your working section and roll a diagonal stroke down and to the right.
• Without lifting the roller, take a diagonal stroke back up and to the left.
• Repeat, forming a large W roughly a metre wide and a metre tall.
• Now fill the gaps between the strokes with vertical passes, overlapping each by about half a roller width, working from the top down.
• Finish the section with a single light, vertical pass from ceiling to skirting to lay the texture in one direction.

That final unloaded pass is what removes the directional marks left by the W. Keep it consistent across the whole wall — always top to bottom, never zig-zag on the finishing stroke — and the wall will read as one flat surface.

Maintain a wet edge as you move along

A wet edge means exactly what it sounds like: the boundary between the section you have just painted and the one you are about to paint should still be damp when you arrive with fresh paint. If it has flashed off, you will see a line. It will not disappear with a second coat either; it will simply sit there under the new paint, slightly raised.

Work in manageable sections, roughly a metre wide, and move sideways along the wall rather than jumping about. Start at one corner and progress steadily. If you are right-handed, working left to right feels natural because you can feather into the wet edge without your arm crossing your body.

A few practical points help here. Paint in cooler parts of the day if the room gets direct sun, keep a window cracked to move air but avoid a howling draught that dries the surface too fast, and never stop mid-section for a cup of tea. If you must break, do it at a corner, a doorway, or a natural stopping point where an edge will disappear.

Avoid overworking the paint

Overworking is the most common mistake made by enthusiastic painters. Once paint starts to set, every extra pass does damage rather than good. It lifts partially dried film, creates roller marks, and leaves a texture that no amount of touching up will fix.

The rule is simple: spread the paint, blend the edge, lay it off once, and walk away. Resist the urge to have one more go at a patch that looks slightly thin. If it genuinely needs more coverage, let it dry fully and address it on the second coat, which is the correct place for that work.

• Don't press hard: pressure squeezes paint out to the edges of the sleeve and leaves tramlines. Let the roller's weight do the work.
• Don't roll back and forth in one spot: two or three passes to blend is plenty.
• Don't chase imperfections: if the wall is uneven, that is a preparation job, not a painting one.
• Do cut in first: brush the ceiling line, corners and skirting before you roll, then roll into the brushed edge while it is still wet.

Getting the second coat right

Most emulsion needs two coats for a reason, and the second coat is where a wall really comes together. Give the first coat the full drying time stated on the tin — usually two to four hours — and don't be tempted to start early because the surface feels dry to the touch. Underneath, it is still soft.

Roll the second coat in exactly the same pattern, in the same direction, working in the same order around the room. Consistency between coats matters more than any single clever trick. When you finish, clean your sleeve properly under warm water and let it dry standing on end; a well-cared-for roller will give you a smoother finish next time, and it saves you buying new ones for every job.

Do those things — a decent load of paint, a W to spread it, a wet edge to blend into, and the discipline to stop rolling once it is laid off — and you will get a finish that looks flat, even and professional under any light in the house.