I have been trying to find advice on how to do something similar to patch a rock lath plaster wall around my thermostat wires. It doesn't look like the original screws were screwed into anything substantial.The area where I'm trying to patch around the hole in the wall to provide a solid surface to rehang the thermostat screws, but after three coats, the patching is still flimsy and won't hold a screw much less the entire thermostat. The guy at the hardware store suggested a fiberglass patch which I used but now I'm thinking I should have used aluminum? HELP?! See photos to make more sense of the problem.
From your original picture, here's how I would have repaired it:
- Use some small pieces of (scrap) wood as strapping. Big enough to form a good support but small enough so you can get them into the (massive) hole for the wires. Somewhere between 3/8" and 3/4" would work best.
- Anchor the strapping with drywall screws. Ideally they can be placed so they're covered by the thermostat. You can fill these in with compound or spackle if you want to.
- Just use regular wood screws (eg, #8 x 1.25" or 1.5") to mount the thermostat to the strapping, no need for drywall anchors.
Cut a square of drywall or wood roughly the size of the original hole. Place it over the original area and trace with a pencil. Then cut out the traced area and glue it in the hole. Then use the fiberglass patch. You need something to hold the screw and the plaster alone will not do.