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my basic question is: what is the proper material and method for repairing defects on the walls in my old house? To give context, I am in Virginia, United States and the house is a concrete block construction Cape Cod style built in 1946, and I believe the walls in question are all original.

My first issue is identification - I'm not even sure "plasterboard" is the correct terminology for what these walls are made of. They are definitely not modern drywall / gypsum board, they are more dense. I don't suspect they are lathe and plaster, but instead some kind of sheet material that seems close to "cement board", with a plaster coat on it. The best clue I have is where there are penetrations through the walls, at outlets and light switches:

exposed cross section of this wall type at a light switch

So what is this really called? I would be willing to drill some exploratory holes (perhaps in a closet) if that helps identify, but currently there are no larger openings or exposed areas. I feel like proper identification will help me track down better resources on this, but for the moment I'm a little lost on what it actually is.

The second and ultimate question is what's the proper material and method for surface repairs on this type of wall? I have built and repaired plenty of drywall before, but am not sure what's appropriate for this type of wall. Is it a problem to use drywall joint compound? That would be the first obvious choice, but I'm wondering if that's detrimental for some reason, like over the long haul it will not adhere properly. Or should I be using some kind of plaster mix?

The types of defects I want to fix are places with some cracking, previous screw/nail holes (from hanging things, some of which were obviously patched with drywall compound by someone previously with little care for detail), and a couple spots where the surface layer is broken off:

larger (~6") surface repair that needs to be done

cracking that needs better repair

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That looks like plaster to me. They would typically (there are many variations on this... a pro-plasterer could be more definitive) apply what is called a brown coat over a substrate (maybe wood lath with scratch coat, maybe button-board, maybe gypsum board, maybe wire, etc.). The brown coat was cement mixed with a large proportion of sand. Then they would trowel on a finish coat of what you might think of as typical plaster.

You should be able to use whatever plaster material you want, including cutting out sections a installing gypsum board and feathering into existing wall sections. You could just skim-coat the entire wall with plaster (after addressing cracks/defects).The key is to properly prepare the surfaces prior.

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  • Oh interesting, so it sounds like the cement-like portion I am seeing in cross section is the brown coat. I have read a little on this traditional 3 coat plaster process but have no practical experience with it. I'll head to the hardware store and look at plaster mix options. But to clarify about the idea of just using drywall joint compound - so is that definitely a bad idea? In what ways would that fail or cause problems?
    – thkemp
    Commented Jan 30, 2021 at 18:08
  • Using joint compound can work. You should be able to patch and blend at will with existing surfaces and/or skim the entire wall area if desired. you need to make sure that existing surfaces to which you apply it are flat (as in no sheen) and clean. Pros like the entire wall skim method for "old wall" situations where aesthetics are critical, because it can be made very flat and smooth (of course, years of experience involved there...)
    – Jimmy Fix-it
    Commented Jan 30, 2021 at 21:19

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