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I'm trying to patch a hole in a hollow core door.

I already filled the internal area behind the hole with expanding polyurethane foam.

My plan was to level it off, apply drywall putty and paint it. ..Unfortunately foam is rather soft; I had expected it to be the hard and crunchy kind. If I put the drywall putty on it and paint it I'm afraid it won't even be able to stand light finger pressure without denting.

So my question is: is there an alternative putty I can use that will have some strength once dried? The polyurethane is strong(ish), it's just soft so I need something rigid enough to distribute pressure.

I'm thinking either wood filler, or PL100 adhesive (perhaps mixed with filler). Another option would be epoxy.

If I need to I can dig out the foam and use a more rigid one. Gorilla glue?

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  • Bondo, which is used for car body repair is a possibility and is used often for a strong wood filler. It is very sticky and sands easily. It might require several coats but it dries fast. Only concern would be the flexing and softness of the hollow core door itself. I can't really think of a better alternative other than replacing the door.
    – Kyle
    Sep 5, 2022 at 7:17
  • Use fiber mesh to hold it together
    – Traveler
    Sep 5, 2022 at 8:29

2 Answers 2

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I'm assuming the door is PVC or painted MDF and it's a reasonably sized hole (20mm+ or 3/4").

You've done exactly what I would have done. No filler would fill the hollow-core door, it would just fall into the middle. You pre-fill with expandifoam to give a base substrate from which the filler can work.

Cut back the expandifoam if necessary with a Stanley knife/utility knife. You want to leave at least a 3mm or 1/8" depression from the surface of the door. It doesn't have to be neat.

Fill the hole with epoxy-based wood filler (might be sold as "High performance" or "exterior" filler; like Ronseal High Performance Wood Filler). They come with the filler and a pouch of "hardener" (i.e. the polymerisation activator or catalyst). Make a big ball of dough by mixing them up to the instructed ratios. Lavishly apply, leaving the surface proud. Don't take your time over it - you have 10 minutes to get it on before starts to "go off". You don't need a good finish. Slap it on.

NB. Wear disposable nitrile gloves. If indoors you should have an A1/A2 respirator (an organic vapour mask; not just a dust mask). Read the warning label.

Wait 20 minutes or so. Check it's solid. Sand back with 60-120 grit paper (feel free to use an orbital or detail sander, it should be very tough). Feel free to give a "second coat" of filler to fill any finer holes if there are any and you want a perfect finish.

NB. P2/P3 particulate mask whilst sanding styrene based filler. Even outside. Read the warning label.

Repaint door ensuring you use a suitable wood primer over the filler.

It should be absolutely rock solid.

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  • 1
    I followed your advice and the repair came out perfectly! Thanks. The product I used was Minwax #21600. You were correct on all points, the filler sets very fast, and it stinks. We had to leave the house for half an hour, and you could still smell it hours later.
    – Drew
    Oct 4, 2022 at 3:40
  • @drew oh I'm so happy it worked. It's not difficult to get wrong, it's just knowing the technique hey. Re stinkiness yes, it's worth taking the door off and doing it outside - this is why I said wear an a1 respirator! It will literally make you keel over if you get too much of it. But the smell should dissipate completely in 24h - it's not like solvent paint which stinks for months.
    – Rab
    Oct 5, 2022 at 7:03
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When I worked in a door factory skin damage was repaired with polyester resin (builder's filler). This stuff has a strong chemical odor so wait for a period of open windows weather.

This, sanding, and painting could restore a damaged door skin to literally good as new. repair doors were sent out as top quality, not seconds.

There's several different brands of this stuff and they're all basically the same, a putty like resin and a and a hardener, mix and apply, once it starts hardening you can shave it down a bit with a knife or saw blade, when it's fully hard you can sand it.

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  • Can you give an example of one? I tried searching "polyester resin builders filler" but I'm mostly getting marine and fiberglass resin product.
    – Drew
    Sep 6, 2022 at 2:34
  • we call it "bog" in AUS and NZ, seems to be the same sort of stuff that they call "Bondo(TM)" in USA
    – Jasen
    Sep 6, 2022 at 6:46
  • @Jasen did the door factory exclusively make PVC doors? Whilst similar, polyester is preferable over polystyrene (as I answered) as it's less toxic. But I'm pretty sure polyester wouldn't bond if the OP's door is MDF/fibreboard/wood. I'd be interested what materials you used it on regardless of OP's door though.
    – Rab
    Sep 6, 2022 at 13:36
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    these were flat hardboard skins, they were also relieved MDF skins were but they were not usually repaired as they had textured surfaces which made repair uneconomic.
    – Jasen
    Sep 7, 2022 at 7:44
  • Well given that nugget of knowledge, I'm going to give polyester filler a go on wood repairs then next time I have the chance, thanks @Jasen. I've got some for car bodywork (Isopron "P38", if that helps you, OP - I think that should be chemically similar to what Jasen used), which I know works great on steel and plastic, but the instructions don't say anything about wood. Epoxy woodfillers are always polystyrene based. But I'm figuring if it sets and its bonded then it's not likely to fail further down the line, hey.
    – Rab
    Sep 7, 2022 at 9:14

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