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I'm installing a door knob and latch in a hollow core door. While screwing in the latch faceplate, I noticed a long hairline crack that starts at the latch bore and runs maybe 8 inches up the stile. Highlighted in red in the picture.

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The crack opens up when I tighten the faceplate screws. I initially thought one of the screws cracked the wood but now I'm thinking the crack was there when I got the door and it's the pressure of the plate that's making it open up. The crack doesn't look like it runs over the screw hole.

This is probably not a big deal but I don't like having the crack there and it's not going to get better on its own. How do I tie it back together? The crack is too tight to get glue in there.

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Many woodworkers use superglue for hairline cracks, but epoxy is also good for this and is stronger.

Superglue is thin, will enter hairline cracks due to 'capillary action'. With no further help expect 1-3mm penetration, enough to glue surface of crack together. If you blow glue into crack like @Graphus mentions in comments, 3-5mm penetration.

If this is not enough for you, use slow-set epoxy. Apply to surface and you can wait >10 minutes for glue to penetrate due to gravity!

The crack opens up when I tighten the faceplate screws.

The crack is too tight to get glue in there.

Tighten faceplate screws to help glue enter crack. After applying glue remove faceplate and crack will close, so no need for clamping.

Then leave glue to set before final installation of faceplate.

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  • When I tighten the faceplate screws, the hairline crack is more noticeable but is still a hairline crack and does not open up enough work anything inside.
    – aquaticapetheory
    Apr 24, 2022 at 16:41
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    @aquaticapetheory, you seem to be thinking of viscous glues. Superglue is typically fairly thin, and water-thin varieties are available if you really need it. Either will readily wick into hairlines cracks all on their own, but there are various tricks to aid the process (with any glue) widely parroted online. At the most basic you can blow the glue into a crack! Without compressed air this works best through a straw (or the barrel of a ballpoint) but you can do it just by blowing through pursed lips.
    – Graphus
    Apr 24, 2022 at 18:05
  • You can also purchase a needle and syringe from your local pharmacy or on Amazon. 20- or 22-gauge should fit. Make sure you're not using "instant" CA glue, as that will harden before you've had time to really place and adjust everything. It may take a couple of injections, you'll just have to see how it performs.
    – MattDMo
    Apr 24, 2022 at 19:41
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Since this is the part of the door that gets the most stress, I'd actually advise not using epoxy or CA glues for this. Epoxy will be harder to get into the voids, and CA glue will either soak into the open pores of the typical softwood construction of these doors or be so thick and fast-setting it will be a bear to work into the split. It'll be pricey, too.

Use a clamp as a spreader and gently open the split up and then use a syringe and/or a thin glue spatula to work as much wood glue into the void as you can. You really want to work as much as you can into the split, working the spatula as deep as you can go. The idea is that you want a fair amount of messy squeeze-out once you clamp it up. Keep adding glue and taking the pressure off the spreader occasionally until you see squeeze-out all along the split.

Be generous with the glue, and take your time. Those little cheap suction cups used to hang doodads on window panes can be used along the split to force glue deep into the void.

Then remove the spreader and clamp the entire stile up firmly. There should be a fair amount of squeeze-out that you can deal with in the usual manner.

Use a wood glue that has a longer set-up time so you can take your time jamming it in there.

You can take a cue from furniture and guitar repair folks and first drill small holes at the ends of the split to try and keep it from lengthening as the door is stressed.

You may want to cleanup the edges of the void where the plate goes in with a router or chisel so the hardware isn't trying to force the same split to open.

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  • I'll try the spreader approach but, having tried to open the crack up with my fingers, i think it would take a lot of pressure and I'm afraid that will do other damage like ripping the hardboard skin off the stile faces.
    – aquaticapetheory
    Apr 24, 2022 at 16:43
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    Although there are very thin epoxies available your typical epoxy can be made very fluid by simply warming the application site first. A hairdryer is all that's needed; relatively gloopy common epoxies at room temperature are instantly changed to a fluid, flowing consistency when they touch the well-warmed wood. I use this trick all the time to ensure proper wetting of a void's surface doing fills, and deeper penetrating into holes and cracks.
    – Graphus
    Apr 24, 2022 at 18:11
  • @aquaticapetheory, if you need to open up a fine crack just a little to help repairing with a thicker glue it's often best to take a direct approach — work directly at the crack. You can insert the edge of a chisel into it, push or tap it in a smidge and the tip will of course act like a wedge. You can't use this trick all the time because sometimes the minor damage caused would be too visible, but this sort of situation is a prime candidate for it. As soon as you withdraw the chisel the wood will spring closed, but by all means add clamping pressure to ensure a better bond if using PVA.
    – Graphus
    Apr 24, 2022 at 18:22
  • @aquaticapetheory this is why I recommend wood glue and suction cups and/or thin glue spatulas. The idea is just keep the crack just open enough for you to work the glue in. The nice thing about wood glue for this kind of fix is how much open time you have to just get the glue into the voids any way you can.
    – user5572
    Apr 24, 2022 at 22:23
  • No pores in softwood!
    – Volfram K
    Apr 25, 2022 at 6:53

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