Why is my paint running and sagging down the wall?

A sag is gravity beating surface tension. The paint film is heavier than the surface can hold, and the causes are all variations of too much paint in one place at one time.

What you'll need

• fine abrasive and a sanding block
• brush or roller
• scraper or blade for heavy runs (optional)

Step by step

1. Catch it while it is wet.A run brushed out within the first minutes disappears completely. Keep looking back at what you have just painted, particularly on edges and mouldings.
2. Work out why it happened.Overloaded brush, coat too thick, over-thinned paint, non-absorbent surface, cold or damp conditions, or spreading too little. Each has a different correction.
3. Load the brush properly.Dip about a third of the bristle length and tap off the excess against the inside of the tin rather than scraping it on the rim.
4. Spread the paint out.Apply, spread across the area, then lay off. The commonest cause of runs on woodwork is applying a heavy load and moving on without spreading it.
5. Apply two thin coats rather than one thick one.This is the answer to most run problems, and the finish is better as well as more durable.
6. Watch non-absorbent surfaces.Gloss, tile, metal and previously painted surfaces do not take paint into themselves at all, so the entire film sits on the surface and sags more readily.
7. Check the temperature and humidity.Cold and damp conditions slow the film forming, giving the paint far more time to move. Below a paint's minimum temperature, sagging becomes very likely.
8. Do not over-thin.Thinning beyond the manufacturer's guidance reduces the paint's ability to hold itself on a vertical surface.
9. Pay attention to edges and detail.Paint collects on the bottom edge of doors and panels and in the corners of mouldings. Brush those out deliberately after painting each section.
10. Fix a set run properly.Let it cure fully, then sand it flat with fine abrasive on a block, feather the edges, and recoat the whole panel or section rather than touching in.

Tips

• Look back at what you painted a minute ago. Almost every run is fixable in that window and nowhere near as easy afterwards.
• Two thin coats beat one thick one on every measure — appearance, durability and drying time.
• Paint collects at the bottom edge of doors and panels. Check and brush out those edges deliberately.

Common mistakes

• Trying to cover in one thick coat — The film is heavier than the surface tension can support, so it sags. It also takes far longer to harden and remains soft and easily marked.
• Sanding a run before it has fully hardened — Soft paint clogs the abrasive and smears rather than cutting, and the sanded area sinks further as it continues to cure. Wait until it is hard right through.

If it doesn't work

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