How do I punch and fill nail heads in trim so they disappear?

The mark on painted trim that catches the light is almost always a filled nail hole that was filled level, then shrank. Getting it invisible is about the sequence, not the product.

What you'll need

• nail punch set
• fine surface filler
• 120 and 180 grit abrasive
• primer
• filling knife (optional)

Step by step

1. Choose a punch slightly smaller than the head.A punch the same size or bigger slips off and bruises the surrounding timber. A shade smaller sits centred on the head and stays put.
2. Punch before the surface is finished.Set all the heads first, then fill and sand as one operation. Doing it piecemeal leaves a patchwork of different filler ages.
3. Set the head two to three millimetres below the surface.Deep enough to give the filler something to key into and a shoulder to hold it. Just below flush gives the filler nothing and it falls out.
4. Hold the punch square and hit it firmly once or twice.Tentative taps let it wander. A firm blow with the punch held square drives it cleanly.
5. Choose the filler for the job.A fine interior filler for painted work indoors. A two-part or exterior-grade filler outdoors, because ordinary filler will absorb water and fall out.
6. Fill proud.Press filler in firmly with the corner of a filling knife, and leave it standing slightly above the surface. Almost all fillers shrink slightly as they dry.
7. Let it harden fully, then sand flush.Sand with 120 grit on a small block, finishing with 180. A block matters — a fingertip dishes the filler and creates the dimple you were trying to avoid.
8. Feel for it rather than looking.Run a fingertip across each spot. Your finger detects a step long before your eye does, especially on white paint.
9. Spot-prime the filler.Filler is more absorbent than the surrounding surface. Without a primer the topcoat sinks into each spot and dries duller, so every nail shows as a matt dot.
10. Paint the whole face.Do not touch in individual spots on a finished surface; the patch will show. Prime the fillings, then paint the full length of the trim.

Tips

• Fill proud, sand back. Filling flush guarantees a dimple once the filler shrinks.
• Check your work by feel with a fingertip and by sighting along the surface at a low angle towards a window.
• Outdoors, punch, prime the exposed nail head, then fill. Bare steel under filler rusts and pushes the filler out.

Common mistakes

• Not priming the filler before painting — Filler is far more porous than primed timber, so the topcoat soaks in and dries with a different sheen. Every filled hole shows as a dull spot in the right light.
• Sanding with a fingertip rather than a block — The finger presses hardest at the centre and dishes the soft filler below the surrounding surface, creating exactly the dimple you were trying to remove.

If it doesn't work

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