You have what's called "gyproc lath" plaster walls. This was a kind of plaster that was produced in the late 1950's and early 1960's. It's the missing link between wood lath plaster and drywall. Basically, those grey panels are 2 feet high and 4 feet wide and are constructed much the same as drywall, only they have rough paper on both sides of a 1/2 inch thick gypsum core, and there are no contoured edges. The gyproc lath panels were nailed up on the studs, and then base coat plaster (the stuff that's mostly sand) was applied over the gyproc panels. Then, a 1/16 inch thick coat of "gauging plaster" was applied over the base coat plaster to form a 3/4 inch thick wall ready for finishing with paint or paper. The thickness of the base coat and gauging coats will become larger right before interior corners because there will normally be expanded metal corner bead at both the interior and exterior corners which requires a bit thicker plaster to bury it in.
What you want to do is:
a) Dilute some white wood glue with water to make solution of paintable consistancy and paint that onto the bare gyproc lath panels you want your base coat plaster to stick to. Allow that glue solution to dry.
b) measure the thickness of the plaster you have to replace, and get some wood strips cut on the table saw at your local lumber yard that are that thickness. Stick them up horizontally onto the area you want to replaster.
c) buy some "Base Coat Plaster" at any drywall wholesaler (yes, they will sell to you because drywall wholesalers don't sell to drywall retailers; everyone selling drywall buys it from the local manufacturer). I prefer Perlite Admix Hardwall made by Domtar, but since I can't buy it in Winnipeg anymore, I'm using "Structolite" made by CGC, which is the Canadian subsidiary of USG (US Gypsum). Basecoat plaster comes in a huge bag and costs half of nothing because it's mostly perlite and even has shreaded newsprint in it to give it bulk at minimal cost.
d) Mix some of your "white glue paint" into some water in a mixing container and add some base coat plaster until you get a thick slurry. It will be thick and should hold it's shape well so it won't slump when you apply it between your screeds. If there is any slumping, remove it with a paint scraper when it's stiff (not still wet, but not yet dry).
e) remove the screeds and fill them in with more base coat plaster.
f) Now, apply the white gauging coat. Nowadays, everyone uses drywall joint compound for the white gauging coat because it's much easier to work with than real gauging plaster. There are three kinds of drywall joint compound, and there will be wording on the bag of powder or box or pail of premix as to what's inside:
1. "Taping" or "Regular" is joint compound with the most glue in it. This makes it stick better to the drywall, but it also makes it dry hard and therefore difficult to sand smooth.
2. "Topping" or "Finishing" is joint compound with the least amount of glue in it. That makes it dry softer and therefore easier to sand smooth.
3. "All Purpose" which is kinda half way between 1 and 2 so the drywall contractor doesn't have to carry around two different boxes or pails everywhere in the truck. He can use the same joint compound for both taping and finishing. This stuff sticks OK, but is soft enough to sand smooth even if you don't have the arms of a mountain gorilla.
Anyhow, I'd use "All Purpose" mud for your gauging plaster. It's immaterial whether you buy it as a powder or as a premix in a box or pail, but be aware that the premix comes so thick that I have some difficulty working with it because I don't have the arms and wrists of a mountain gorilla. You're going to want to thin your premix to make it EASY to spread with a trowel. The reason why it's so thick in the box or pail is that neither the manufacturer nor the retailer nor the customer wants to pay for the cost of shipping water. The manufacturer knows that the end user can thin it to his liking at his end.
To make a simple tool for mixing drywall joint compounds, go to any shop where they repair small kitchen appliances, and ask if they occasionally throw out kitchen mixer blades. Most of those places throw them out all the time. Ask them to throw some your way. Mount a kitchen mixer blade in a corded or cordless drill and you can mix drywall joint compounds quickly and easily. Ditto for mixing base coat plaster.
The two ways to put on a uniform thickness (ie: the verb "skim coat") of drywall joint compound over your base coat plaster area as follows:
Alpha): Use a "V" notched adhesive trowel to spread drywall mud onto the base coat plaster, allow to dry, and knock of any blobs that are sticking up above the tops of the ridges. Now, hold the trowel upside down and fill in the valleys between the ridges with more drywall mud using the un-notched edge of the trowel, or
Beta): Use a V notched adhesive trowel to spread drywall mud onto the base coat plaster, and without letting the joint compound dry, mist the trowel ridges on the wall with water using a spray bottle, and trowel the ridges down smooth using the un-notched edge of the trowel. You can also mist water onto the trowel you use to spread the joint compound smooth. This method lends itself to two people working together; one spreading the joint compound using the notched trowel, and the other flattening it down with an un-notched trowel.
Both methods work. Some people prefer doing it one way, some the other. Whatever cocks your Glock.
Finally, after allowing sufficient time to dry, hold a bright light close to the wall but some distance from where you're working to cast light at a sharp angle to the wall. This will make your wall look rougher than it really is. Sand it lightly to make it smoother. When it looks reasonably smooth under such critical lighting, it'll look like it was done by a pro under normal lighting. Repeat the process and sand smooth again. When it looks smooth under critical lighting, it'll look perfect under normal lighting.
Prime. Paint.
I have this kind of plaster throughout my building, which is a 21 unit apartment building. And, I've been repairing these kinds of plaster walls for about 24 years now.
You should also be aware that many people prefer drywall because they think it's easier to repair. Once you know how to repair both, then both are equally easy to repair; one just takes more drying time than the other. But, because plaster walls are heavier, they make for a quieter house. That's because when a sound wave hits a wall, it causes the wall to move, and that movement re-creates a new sound wave on the opposite side of the wall. The greater the mass of the wall, the less it moves in response to the original sound wave, and so the reproduced wave is smaller in amplitude (pronounced "quieter"). This is the reason why there are so many noise complaints in modern buildings built with light weight construction materials like metal studs and 3/8 inch drywall walls. You can hear people talking in different apartments or hotel rooms with walls like that, but that would have been unimaginable 50 years ago and earlier, when all walls were plaster. Masonary walls, like brick and block, have more mass still, and make for even greater noise attenuation between rooms. If you're interested in learning more on this, Google "The Mass Law", which is one of the laws of accoustics.
