We moved house and hit a small, annoying problem. The new place already had a hole in the laundry door for a pet door, but it was 110mm too short for our dog's existing door. The dog was not getting any smaller and nobody buys a new door over 110mm. So we raised the opening, and the fix for the gap left behind is the interesting bit: the piece we cut out of the top went straight back in at the bottom, glued and filled with one product.
Total materials for the repair side of the job: one cartridge of 1 hour cure wood repair epoxy. Here is the whole job, photographed as we did it.
The problem: right hole, wrong height
Pet door frames are not standard sizes. If the previous owner had a smaller dog, the opening in the door will be too short for your door insert, and the sensible place to add height is usually the top, where the frame lines up with your dog's shoulder height. That leaves a rectangular hole below the new opening that has to be filled so the frame has something solid to clamp onto, and so the door does not whistle, leak and look like a patch job.
Why the offcut is the perfect fill piece
When you cut the opening higher, do not throw the offcut in the bin. It is the exact thickness of your door, it has the same face skins, and it is already painted in the right colour. No sheet of ply from the shed will match a door skin as well as the door's own material. Cut it to the size of the gap and most of the work is already done.
Step by step
1. Mark and cut the new opening
Measure the pet door frame, mark the new top line, clamp a straight edge and run a circular saw along it. Score the paint line with a knife first so the skin does not chip.
2. Size up the gap
With the opening raised 110mm, you are left with a 110mm strip to fill at the bottom. Clean, straight edges here make the glue joint tight, so tidy the cut with a sanding block before you fit anything.
3. Test fit the offcut
Cut the offcut to the width of the gap and dry fit it with clamps. You want a snug slide fit, not a hammer fit. Any small gaps left over are fine because the epoxy fills them in the same pass as the glue up.
4. Butter the edges with epoxy
Mix the two parts of a 1 hour cure wood repair epoxy and spread it over every mating edge of the fill piece with a plastic spreader. Be generous. This is the step where using a structural epoxy instead of a gap filler pays off, because the same bead is doing the gluing and the filling at once. On a hollow core door like this one, the epoxy also bridges between the two face skins, so the filled section ends up stiffer than the rest of the door.
5. Glue it in and fill the joint
Push the piece home, clamp it flush with the door faces and run epoxy along the joint lines on both sides. Squeeze out is good. Fair it off with the spreader while it is soft and you will save yourself sanding later.
6. Sand back an hour later
This is why we used the 1 hour cure rather than a slower product. By the time you have cleaned up the tools and had a coffee, it is hard enough to sand. Block sand the joint flush, feather the edges out wide, and dust off.
7. Prime, paint, fit the frame
Spot prime the repair, paint the door and screw the pet door frame into the raised opening. The frame covers the top and side cuts, and the filled strip below disappears under the paint.
That is the finished door. The join sits about 100mm below the frame and you cannot pick it.
Why epoxy and not builders bog or caulk
A door gets slammed. It flexes, it takes knocks from the dog, and the filled strip sits right where paws land. A rigid polyester bog is not an adhesive, so the fill piece would be relying on friction and hope, and the joint line would crack the first dry summer. A two part wood repair epoxy bonds the piece in structurally, stays slightly flexible when cured, and sands and paints like timber. One product did the gluing, the gap filling and the surface fairing on this job.
If you want the full comparison, we have written up when a filler is fine and when you need a repair epoxy, and there is a customer sill repair that uses the same splice and epoxy method on rotten timber.
What you need for this job
- DT Wood Repair Epoxy, 1 hour cure. One cartridge did this whole door with plenty left over.
- Heavy duty epoxy gun if you are doing more than one cartridge, or the repair is part of a bigger restoration.
- Circular saw, straight edge, clamps, a plastic spreader and a sanding block.
FAQ
Does this work on a hollow core door?
Yes. This door is hollow core. The epoxy grips both face skins and the edge material, and the cured joint is stiffer than the door around it. On a solid timber door it is even easier.
Should I use the 1 hour or 4 hour cure?
For a join like this, 1 hour. The joint is small, you want to sand and move on the same day. The 4 hour cure suits bigger fills where you need working time.
What if I did not keep the offcut?
Any board close to the door's thickness works. Ply or MDF cut to size, glued in the same way, then a skim of epoxy over the face to level it. The offcut just saves you the thicknessing and matching.
Will the join show through the paint?
Not if you feather the sanding well past the joint line and spot prime before top coating. The joint in the photos above runs the full width of the door and is invisible after two coats.