I'm not an expert, but have done a lot of repair work over the years. Two methods have worked out well for me. Wet the wall down (moist not dripping) apply a coat of one step. I usually choose a type with a low minimum application thickness for patches, name brand, they have two main types here (Knauf Redband and Goldband) with slightly different minimum and maximum recommended thicknesses and slightly different characteristics (the coarser, Goldband, is easier to work with). You can fudge the maximum recommended thickness a bit if necessary, it just gets harder to work with. Lightly spray the finshed Patch/wall with a pump sprayer (old Windex bottle) a couple of hours after your finished and then again every hour or so, so it sets slowly.
Think big, the entire patch as a unit, find something that reaches past the edges, use this to spread your plaster after you've found some way to get it on the wall
. Trowel (smooth) as little as possible after it is level. Often the more you trowel, the more uneven it becomes. Don't even try until it has started to set a little.
Second method, I most always use a (spray on) deep sealer anymore (let it dry overnight), for the simple reason it slows down moisture transfer between the existing wall and your repair and can help rebond the edges (capillary action). And use a brush on bonding agent, if the existing plaster still seems iffy.
Preparation is usually to lightly tap the borders of the area to be patched, with a straight claw, body or roofing hammer, to knock off the loose stuff. The wedge or pointy end not the flat end you nail with. This is where the fun starts, you don't really know how far the damage has spread. A 2'X2' hole can turn into an 8'X8' hole, pretty quick. I use a wire brush to knock off the loose material after the hammer or a very stiff nylon scrub brush. Get the dust off with a vacuum and damp rag.
A brown coat or a bond coat, then a skim coat, may last longer or the rest of the wall may develop trouble first. In my experience the one coat outlasts the existing plaster drastically.
A good solution, other than taking the whole wall down to stone, is a sturdy wall paper. They make stuff that is basically, fiber glass and nearly indestructible (works like drywall tape, just in a big way). The wall becomes a unit. Or fur the wall and put up Sheetrock. The fur and Sheetrock, along with a layer of insulation reduces heating costs and is a do it once solution.
The main trick with plaster is the proper working consistency. Think Ready Whip, not cement. Pre mix may be the way to go for a small repair.
We have a lot of trouble here with the brown coat and the skim coat separating, they say it's because of mold (damp climate) and/or improper application. Brown coat takes a long time to dry, bonding plaster is quicker (hours), one step a couple to a few days. Drywall mud works well to fix the small mistakes or feather the edges of your patch. The drywall mud I use has a fairly high fiber content, likely some cellulose and sticks like glue.
Like I've said, I'm not an expert, but some/most of my patches are 25 years old and still there. Sometimes it is hard to tell just how extensive the problem is, a repair may not last, the surrounding plaster may let loose. If in doubt, furring and Sheetrock is likely the easiest and longest lasting solution.
Sorry I wrote a book, had to explain in a blurb.
