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Why paint misbehaves in the first place

Nearly every drip, sag and brush mark comes down to one of two things: too much paint on the surface, or not enough patience. Paint is designed to be spread thin and worked into the surface, then left to level itself. When it goes on thickly, gravity wins and it slides. When it's brushed back and forth after it has begun to set, the bristles leave tracks that never flatten out.

Water-based paints are the usual culprits because they skin over so quickly. A modern quick-dry eggshell or trim paint can be touch-dry in half an hour, which gives you a very short window to lay it off properly. Traditional oil-based gloss gives you far longer to work, but it sags more readily because it stays soft for hours. Knowing which one you're holding changes how you should behave.

Sorting out drips, runs and sagging

A run is paint that has slid downwards before it set; a sag is the same problem caught earlier, when the film has thickened in a patch but not yet moved. Both are thickness problems.

If the paint is still wet, you can rescue it. Load a dry brush with almost nothing and brush the excess out sideways, feathering the edges into the surrounding film. Work quickly and lightly. Do not go back over it a second time — you'll only make it worse.

If the paint has started to skin, leave it alone. Touching a part-set film drags it into ridges. Let it dry completely, then:

• Scrape the raised section flat with a sharp scraper held almost parallel to the surface.
• Sand the area with P180 paper, feathering well beyond the repair so you can't feel a step.
• Spot-prime any bare timber or filler, then repaint the whole panel or section — not just the patch.

Prevention is simpler: unload your brush on the inside of the tin rather than wiping it on the rim, roll off excess on a roller screen, and paint in sections you can finish before the paint starts to set.

Beating visible brush marks

Brush marks are mostly a technique issue, though paint consistency matters too.

• Use the right brush. Synthetic filaments suit water-based paints; natural bristle works better with oil-based. A cheap brush will leave marks no matter how careful you are.
• Dip only the bottom third of the bristles. Overloaded brushes flood the surface and leave ridges at the end of every stroke.
• Lay off with the tip only. After spreading the paint, finish with light strokes in one direction, from the dry area into the wet, using just the very ends of the bristles.
• Don't over-brush. Two or three passes to spread, then one flowing stroke to lay off. More than that and the paint starts to drag.
• Keep a wet edge. Work from one end and never let a section dry before you join the next one to it.

If the paint feels heavy or drags, thin it very slightly in line with the manufacturer's instructions rather than adding a splash of water by eye.

Prep and conditions do most of the work

Most paint faults are decided before the tin is even opened.

• Wash surfaces with sugar soap and rinse thoroughly. Grease and dust cause crawling and patchy adhesion.
• Sand glossy or previously painted surfaces lightly to give a key.
• Prime bare timber, filler and patches. Bare surfaces pull moisture out of the paint at different rates and leave a dull, uneven finish.
• Work at between roughly 10°C and 30°C. Cold paint thickens and won't level; a hot room dries it before it can.
• Stir the paint properly rather than shaking it. Shaking traps air, which shows up later as bubbles and pinholes.
• Ventilate, but avoid a direct draught or blazing sun on the wall — both cause the surface to dry faster than the film beneath it.

Rescuing a finish you're not happy with

If you stand back and the wall or door looks streaky, don't immediately blame the paint.

Fine nibs and light brush marks usually disappear with a light sanding using P240 paper and one more thin, carefully applied coat. For a whole wall that looks patchy, the likely cause is a roller sleeve that's too long-pile, or a coat spread too thinly in places. Try a short-pile sleeve on smooth surfaces and keep the pressure even across the wall.

Where a run has left a hard ridge, sand it flush, spot-prime and repaint the entire panel. Painting just the repair always shows, because the sheen and texture won't match the surrounding finish.

And if all else fails, one more coat applied slowly, with a decent brush and a genuine willingness to stop fiddling, will fix more paintwork than any product on the shelf. The secret isn't speed — it's leaving the paint alone once it's on the wall.