How do I repair a hole in a hollow core door?

Filling a hole in a door with nothing behind it, using expanding foam as a backing, and being honest about when replacement is cheaper.

Difficulty: intermediate
Time: 2 hr
Read: 4 min

Short answer

Trim the hole square, push crumpled paper in as a former, fill the void with low-expansion foam, cut it back below the surface once cured, then build the face up in two or three thin coats of fine filler.

A hollow-core door is two 3mm skins over a cardboard honeycomb. Repairing a hole means creating something for the filler to sit on, because there is nothing there.

What you'll need

A utility knife
Low-expansion filler foam
Fine surface filler
A filling knife and a flexible spreader
Abrasive paper, 120 and 180 grit
Primer and paint

Step by step

Decide whether it is worth repairing.A new hollow-core door costs very little. Repair makes sense for an unusual size, a matched set, a moulded panel design or a door you cannot get again.
Tidy the hole.Cut back the broken skin to sound material with a sharp knife, undercutting slightly so the edges are not feathered. Ragged edges telegraph through the finished paint.
Remove the loose honeycomb.Pull out the crushed cardboard around the hole so the foam has clean space. Do not go far — you only need 20–30mm around the opening.
Push in a former.Crumpled newspaper or a scrap of card pushed 15mm behind the skin stops the foam disappearing into the whole door and gives it something to bear on.
Fill with low-expansion foam.Fill to slightly below the skin, not level. Foam expands substantially, and foam that swells proud of the face has to be carved off, which risks the surrounding skin.
Let it cure fully.Several hours at least, longer in a cold room. Cutting foam that is still soft in the middle produces a spongy base that moves under the filler.
Cut the foam back below the surface.With a sharp knife, 2–3mm below the door face. That depth is what the filler will occupy.
Fill in thin coats.Two or three passes of fine surface filler, each allowed to dry, rather than one thick one. Thick filler shrinks in the middle and cracks.
Sand beyond the repair.Feather the edges well out into the surrounding door with 180 grit on a block. The visible edge of a repair is almost always where the sanding stopped.
Prime the repair before painting.Filler is far more absorbent than the door face. Without a primer coat the repair shows as a dull patch through the topcoat.
Paint the whole face.Not just the repair. A painted patch always reads as a patch, however good the filling was.

Tips

A pre-cut plywood or MDF patch glued behind the skin, then filled over, gives a far stronger repair on a door that will get knocked again.
For a door with a moulded panel profile, matching the shape is the hard part. A silicone mould taken from the same profile elsewhere on the door works remarkably well.
Two-part car body filler sets fast, sands beautifully and is stronger than decorator's filler for the final surface on a door that takes knocks.

Common mistakes

Using standard expanding foam — Standard foam expands with real force and can bulge or even split the door skins from the inside. The door then has a permanent dome that cannot be sanded out. Use low-expansion foam only.
Filling the hole in one thick pass — Deep filler shrinks unevenly as it dries, sinking in the middle and cracking at the edges. It looks fine until the paint dries and then the repair appears as a dished patch.

If it doesn't work

Repair visible as a dull patch after painting

Cause: Filler not primed — Fix: Prime the filled area, then repaint the whole face.

Repair sunk after drying

Cause: Filler applied too thick — Fix: Apply another thin coat, sand flush, and prime.

Foam bulged the door face

Cause: Standard rather than low-expansion foam — Fix: Cut it back below the skin, and accept that a bulged skin may not come fully flat.

Edge of the repair shows in raking light

Cause: Sanding stopped at the edge of the filler — Fix: Feather the filler well beyond the repair with a sanding block.

Questions people ask

Is it worth repairing a hollow core door?

For a standard size, a new door often costs less than an hour of your time plus materials. It is worth repairing when the door is a non-standard size, part of a matched set, or has a moulding you cannot buy.

Will the repair hold up?

For a hole in a low-traffic area, yes, for years. A foam-and-filler repair has no real strength, so a glued plywood backing patch is worth adding where the door will get knocked again.