Many people believe that "plaster wall repair" is an oxymoron. They feel that plaster walls cannot be repaired.
It is true that plaster walls can be difficult to repair--especially if they are too far gone. Like rust on a car, you need to strike at the first sign of problems.
Fortunately, you do not need a special plaster repair kit. All you need are simple drywall tools that you can easily and cheaply obtain at a home improvement store.
Why Plaster Is Harder to Fix
With drywall, it is often more expedient to rip out entire sections and replace with large sheets.
As difficult a material as it is, drywall does offer some advantages over plaster when it comes to repairs. It is possible to remove only the section that needs fixing (plus a few inches beyond), without the entire wall collapsing. The other advantage is that drywall has no backing. Once you cut through drywall, there is nothing behind it except for studs and insulation. It all removes in one layer.
By contrast, plaster walls are made of two layers: the outer plaster and the inner wooden lath.
With plaster, your best bet is to preserve whatever is there and fix it, rather than tearing it out. The moment you start tearing out chunks of plaster, it becomes a never-ending process. One chunk leads to another, and before you know it, you are hauling plaster and lath to your backyard for disposal..
1. Score the Crack
Using a putty knife, a 5-in-1 tool, or a dull utility knife, score the hairline crack to open its edges. This may seem counter-intuitive, but you need to increase the area for the repair compound to stick to.
Brush off any loose crumbs.
2. Spread Joint Compound
With a wide taping knife (of the type used for drywall), smooth a thin layer of joint compound over the cracked section.
Press paper tape or fiberglass tape into the wet area, along the length of the crack. This is ordinary drywall tape. You do not need to purchase any special plaster repair tape.
4. Curing, Feathering
Let fully dry. Feather joint compound over taped area so that the compound extends two or three inches past the taped area.
5. Second Curing
Let the joint compound dry. Lightly sand it down with fine sandpaper to get rid of any bumps or ridges. Do not sand so hard that you dig into the tape.
6. Second Feathering
Feather a second layer of joint compound, this time extending the edges even farther to about six to eight inches. Let dry. Sand.
7. Finish Coat
Finally, your third coat of joint compound takes the edges out to twelve inches. Since this is your last chance to get the sanding right, be careful to make it smooth.