Drywall – Remodeling a pre-1920’s home with planks on the interior walls

Removing plaster can make tremendous amounts of dust, isolate the area with plastic to cut down on the spread, and get yourself a dust mask and a helmet is a good idea. Cover any cold air returns and heating vents. Once you're prepared, taking plaster walls down is pretty easy, whack it with a hammer to bust a hole, then you can usually get a shovel or pitchfork behind the plaster and it will pull away from the lathe easily as you break the keyway that holds it in. Clean up all the plaster and dust, then go after the lathe with a crowbar and/or hammers; then start cutting the vertical studs with a sawzall.

Double, triple, quadruple check that it's not load-bearing, if the blade is binding when cutting through a stud, there's downward pressure, and even if it's not structurally load bearing, things could've shifted over time above, and taking it out might make things drop or sag.

From your description of putting a screw into the walls, it certainly sounds as if you have lath-and-plaster. Another sign is the color of the dust: with drywall, it will be very white and uniform, whereas with lath-and-plaster, it's greyish and has darker flecks.

A lot of stud finders work by detecting the change in the dielectric constant of the wall as you move horizontally across the wall. Stud finders have a hard time with lath-and-plaster because the plaster is solid and can often be 1/2" thick, then underneath the plaster you there are only small gaps between the wood lath, so you never really get the sharp change in capacitance that they look for. On the other hand, drywall is spongy so there's less material between the stud finder and the stud that it's supposed to be sensing.

I had a deep-scan stud finder, and it wasn't that reliable, but I developed a technique that I had reasonable success with: do several scans at different heights and different starting points on the wall, noting where it said there was a stud. I also found that scanning slowly was better than faster. Eventually, I would get to an "average" location that was a pretty good guess.

I later learned (at least in my house) is that there's a stud on each interior wall right where it runs into an exterior wall. I could then mark 16", 32", etc. back from that stud and be OK.