Any new wiring or plumbing done on an older home usually requires getting behind lath-and-plaster walls. If an entire wall section needs to be removed (such as for new kitchen wiring and plumbing), it is easier to remove all the lath and plaster and hang drywall in its place. But if only a small section of the wall needs to be repaired (such as for rewiring an existing fixture or cutting in a new outlet), I use the following approach, which leaves the lath in place.
To minimize damage to the walls, I score the wall with a chisel around the work area. Then I remove the plaster inside the scored area by gently tapping with a trim hammer. Once I’ve got the loose plaster out of the work area, I make a patch out of 1/4-in. pegboard and orient it with the textured side out. The pegboard’s holes serve several purposes: They allow me to eyeball the patch size so that I can sketch its outline while holding the material in place; they give me countersink locations for drywall screws; and they serve as keyways for the topcoats of mud that will finish the patch.
I cut the patch with a jigsaw, then screw it to the lath, shimming as necessary to leave the patch just shy of flush with the surrounding plaster. This section of wall is now bound together more solidly than before. To finish, I tape around the perimeter of the patch, bridging the pegboard and the original wall. Now I can apply several coats of joint compound to feather the patch into the wall.
By the way, you’ll get better adhesion if you score the original wall with rough sandpaper prior to floating the mud. And if you have large gaps in the wall between your patch and the existing plaster for any reason, use a quick-setting “hot mud” to fill these large spaces first. Hot mud comes as a powder that you can mix with water on site. Setting times vary from 20 to 60 minutes, which really speeds things up.Conan Bliss, Boulder, CO