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We just moved into a ranch house that's well over 100 years. Unfortunately our family made some unusual improvements to it while the last five generations were living there (during various decades).

In the master bedroom it was decided that there was more than enough room to add a bathroom WITHIN the footprint of the master bedroom. They accomplished this by basically framing-out a closet with wood studs and using plywood instead of drywall.

HERE'S THE PROBLEM: I just removed the huge medicine cabinet/mirror/outlet-strip/built-in-vanity-light so I can install a modern mirror and light fixture. Underneath it, I discovered an unwanted surprise. It appears they had used a saw of some sort to cut a huge 6" hexagonal hole on the far left side of the plywood wall, then the exposed wires had been run out of this hole and spanned about 12" behind the "mirror" to it's hard-wiring source at the top.

So now I have a giant hole to patch/fill in a plywood wall that I can't replace.

The hole isn't going to be hidden by the new mirror and is too weird of a spot to hang a picture.

How can I patch this hole? It's going to be painted but I was told that spackle won't stick to the plywood and will crack when it dries. Plus it's way too big a hole to simply spackle.

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    If you want drywall, why can't you put the drywall over the plywood? A little extra sound proofing never hurt a bathroom. – lqlarry Feb 5 '12 at 3:45
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The process for patching should similar to drywall, but with slightly different materials. I'd suggest using wood filler at the joint.

In more detail: first, get a new piece of plywood of the same thickness as the existing wall board. Then you need to attach your patch piece to structure. For this, you have two options.

  1. Cut away more of the existing plywood until you've exposed some structure you can fasten into.
  2. Cut some lumber to span the opening, and attach it using screws through existing plywood around the opening. Then cut your patch and mount it to the spanning pieces.

If the hole is only 6" wide, a small piece of 2x4 or even 1x2 and a plywood patch will probably cover you.

To seal the seams between your patch and existing plywood, use wood filler. Fill with a putty knife, sand flat, prime and paint. Assuming you can find a 4ft x 2ft plywood panel of the appropriate thickness, total materials cost should be around $20-$30.

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Process is similar to but not the same as drywall -- you patch with the material similar to what the surface is made of.

In my case, I had to patch holes made by an electrician hunting between consecutive ceiling joists for active wires. The electrician used a hole saw so I had the discs to put back in, albeit now slightly smaller. If you're just repairing a hole without the original piece, you'll need to make your own shapes to fill using a similar material of identical thickness.

How to attach it? Rather than cut further through your surface to find a stud or joist to attach a new piece of wood to, consider just attaching another piece of wood "behind"/inside the surface, and then just attaching your replacement plywood to that.

Depending on the size of the hole you're repairing, you may need a thicker/stronger piece of backing wood. For my 6" disc holes, I used 1/2" thick x 3" wide x (disc diameter + 3") long pieces of wood. I used a table saw on a 2x4 to cut the strips, then a mitre/chop saw to cut to length. If I didn't have those saws, I could've used painter shims as a stand-in.

Some people will tell you to use screws to attach everything - the backing wood behind the surface, and your replacement surface itself. This generally works for drywall because the screw can pull itself through the material a little bit and sit flush with the drywall. This is important because you need to eventually sand the surface as flat as you can make it. Plywood is tough and unfortunately doesn't work that way -- usually you're going to need to counter-sink the holes to get the screws to sit flush.

The more holes you have to make, the more work you have to do.

Alternatively, just use construction-grade adhesive. Any hardware store will have tubes of the stuff, fits right into a caulking gun. Glue the backing piece(s) behind the surface first, wait for them to set. Depending on the orientation of the surface, you might be able to get away with gluing your repair piece as well. In my case, I was repairing a ceiling so I could only glue the backing pieces -- still had to make one hole + countersink to hold the repair wood in place. I could have also used glue to stabilize the repair pieces, in addition to the one screw. Remember: fewer holes == less work.

Lastly, you'll need to use sandable wood filler to fill in the seams around the edges of your repair piece as well as over any holes/indentations you made attaching it. Use a putty, spackling, or mudding knife -- or a flat-edged piece of wood if you don't have any of these -- to relatively smooth the surface, then sand flat, prime and paint as necessary.

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  • Hello, and welcome to Home Improvement. Thanks for the answer; keep 'em coming. And, you should probably take our tour so you'll know the details of contributing here. – Daniel Griscom Jun 26 '20 at 16:23
  • Welcome indeed. Note that this is an 8 year old question... Also, most screws with a taper from the shank to the head will countersink themselves into plywood - especially decking type screws that have serrations under the heads explicitly for that purpose. If you're gluing your support piece in, you can screw a similar piece across the outside of the hole to the backing piece on the inside to hold it in place while the adhesive cures, then unscrew and leave your backing in place. Minor details to an otherwise very good answer. – FreeMan Jun 26 '20 at 16:53

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