A hollow core door is not solid wood that happens to be light. It's two thin veneer skins — about an eighth of an inch each — stretched over a cardboard honeycomb. That's why a doorknob, a kicked boot, or a piece of furniture swung too wide goes straight through it, and why you can't just trowel filler into the hole: there's nothing behind the skin for filler to grab. The foam becomes the backing. Everything after that is ordinary patching.
Trim the hole back to firm veneer. Cut away every cracked, splintered, and flexing bit of skin until the edge of the hole is solid all the way around. Filler and foam both need that firm rim to hold onto. Slightly undercut the edge — angle the blade so the hole is a touch wider on the inside than at the surface — and the patch locks in instead of popping out later.
Fill the hollow with foam. Tape off the door around the hole, shake the can, and fill the cavity in short bursts until it's about half full. The foam expands on its own from there. Resist the urge to top it off — an overfilled door is a worse problem than an underfilled one, and you can always add a second shot after the first cures.
Let it cure completely. Follow the time on the can, and don't rush it — foam that feels dry on the surface can still be soft in the middle. A patch skimmed over uncured foam sinks as the foam finishes moving.
Trim the foam just below the surface. Slice the bulge off with the utility knife, then shave it down so the foam sits a hair below the door's face — a sixteenth of an inch or so. That shallow dish is what the filler occupies. Foam left proud of the surface will telegraph through every coat after it.
Skim with filler. Press the filler firmly into the foam's pores with the knife, then pull a tight coat across the whole patch, feathering the edges onto the surrounding veneer. Two-part wood filler makes the hardest, most dent-resistant patch; setting compound sands easier and is fine away from edges and corners. One thin coat usually does it — a second pass after sanding catches the pinholes.
Sand it in two passes. Start with 120-grit to level the patch flush with the door, then 220-grit to take out the sanding scratches. Wipe the dust off with a damp cloth and run your palm across it: your hand finds ridges your eyes miss, especially the patch edge.
Prime, then paint the whole door. Filler and foam drink paint differently than veneer, so prime the patch first — otherwise it shows through the finish coat as a dull, flat spot called flashing. Then paint the entire face of the door, not just the repair.
This repair has honest limits. If the hole is bigger than a dinner plate, if the damage runs into the door's edge or the block where the latch mounts, or if the door is warped on top of being holed, patch it and you'll own a wavy, fragile door that announces the repair anyway. A new hollow core slab is one of the cheapest doors in the store — often under $100 — and swapping a slab is an afternoon of hinge and knob work. It's the same judgment call as a big drywall blowout: the fix in how to patch a large hole in drywall works until the damage outgrows the material, and doors are no different.
Spackling straight over the hole. With nothing behind it, the filler cracks at the edges, then caves in the first time something leans on the door. The foam backing is the whole trick.
Grabbing the regular foam can. High-expansion foam against thin veneer can bow the door skin outward as it cures. You'll trade a hole for a bulge — same door, newer problem.
Skipping the primer. Unprimed filler flashes through paint as a dull patch that catches the light at an angle. One coat of stain-blocking primer is five minutes; repainting the door twice is not.
Sanding too soon. Filler that isn't fully hard gums the sandpaper and tears at the edges. Give it the time the container says. Once the patch is sound, a door that also refuses to stay shut is a separate, simpler fix — see door won't latch? how to fix it. And small dings that didn't break through need nothing more than the approach in the easiest way to patch a small hole, filler alone.
A hole in a hollow core door looks fatal and isn't. Trim to solid veneer, back the void with low-expansion foam, skim, sand, prime, and paint the whole face — about an hour of actual work, split by one honest wait for the foam. Keep the damage small and away from the latch, and nobody will ever find the spot. Big or structural damage? Buy the slab and spend your Saturday on something better.
Most holes can be repaired. The foam-and-filler method handles fist-sized and doorknob-sized breaks well. Replacement makes more sense when the hole is very large, the door's edge or latch block is damaged, or the door is warped — a new hollow core slab is inexpensive enough that hours of patching stop paying off.
Two-part wood filler (the auto-body-style kind) cures hardest and resists dents best, which suits a door that gets touched and bumped. Setting-type joint compound is easier to sand and works fine for patches in the middle of the door. Either way, the filler needs the cured foam behind it — filler alone over a void will crack.
The wrong kind can. Standard high-expansion foam pushes hard enough while curing to bow or crack the thin veneer skins. Low-expansion window-and-door foam is formulated to cure soft and gentle, and it's the only kind that belongs inside a door.
A basic prehung or slab hollow core door commonly runs under $100 at home centers, with mid-range styles costing more. If the frame and hinges are fine, a slab swap reuses them — you're paying for the door, a knob if the old one doesn't transfer, and paint.
Sometimes. The area around the knob has a solid lock block inside the door, so a patch there has something behind it. But if the latch no longer holds or the wood around the bore is split, the repair won't restore the door's grip on the latch — that's a replacement, not a patch.