Many people paint or paper over such cracks to make them disappear, but these half-measures don’t eliminate the fissures, which will eventually reappear.
Contractors tend to suggest various radical approaches as a solution—even completely removing the plaster and lathe, and replacing it with gypsum drywall. A much less damaging, cost-effective approach is to re-anchor the plaster with special screws and apply a nylon mesh fabric embedded in a thin coat of new plaster. Woodwork reveals are preserved, trim work isn’t disturbed, and it’s even possible to add your own color to the new plaster and combine repairs and finishing into one operation.
Getting Started Repairing Plaster Walls
First, inspect the walls for any obvious issues that could have caused the cracks, like sagging beams or spreading walls. Old houses usually settle slowly over the years, causing stress that results in those familiar diagonal fissures over door and window openings. A horizontal crack can indicate lateral movement, a sign of structural problems that need addressing. Determine whether the settling is active by traversing a crack with tape and checking for signs of movement after several months. Inspect the supports in the basement and attic areas, looking for fissures in the foundation. If the cracks have been there a long time, they’re often the result of slow settling and can be repaired without structural intervention.
Once you’re satisfied that there’s no active movement, it’s time to address major cracks. Areas that have pulled away from the backer wall must be re-anchored with drywall screws set through 1" square cardboard washers (these help keep the screws from going too deep; you want to snug up to the plaster, not punch through it). For really damaged areas, apply screws about 4" apart.
Next, prepare the wall for the new plaster skim coat. You’ll be trowelling a 1⁄16"- to 1⁄8"-thick layer of new plaster onto the wall, so set yourself up with a temporary bench with plenty of room for tools. Cover the floor with a drop cloth, because lime plaster will stain.
Clean the wall thoroughly with soap and water, then roll on a plaster bonder, which not only bonds the fresh plaster to the old surface, but also controls how much moisture the substrate pulls from the fresh plaster. Too much suction robs the new plaster of moisture needed to mature properly, resulting in shrinkage and a weak surface. Too little suction, though, and the plaster won’t bond—it will be like applying plaster to ceramic glazed brick. So the plaster bond coat provides the glue and setting conditions for the new plaster.
Once the plaster bond dries (give it an hour or so), you’re ready to plaster the wall. Start by mixing finish lime in a 5-gallon bucket, using a 1⁄2" drill with mixing blade [A]. Add your pigment now, too, but don’t be fooled by appearances—the putty will be much darker than the finished plaster on the wall, so be generous with pigment (but keep it below 5 percent of volume). The putty consistency should be akin to brick batter that can be held on a trowel sideways. This putty gets better with age—in fact, old Italian fresco artists insisted on five years’ storage before use—and will store forever in a covered plastic bucket.
Think of your pigmented putty as a bucket of thick paint that needs a hardener added before use. Place some putty on the plywood work surface, and make a donut shape with a reservoir in the center for water [B]. Add gauging plaster—this is the stuff that makes the lime putty set up hard and durable on the wall. Sprinkle the gauging plaster in the water ring, adding enough to equal about half to a third of the volume of the lime putty, then mix the gauging in the water until you have a stiff batter [C]. Finally, mix everything together on the board with a short trowel.
Once you have mixed the gauging plaster with the lime you need to work deliberately, as you’ll have about 30 minutes to work it onto the wall and trowel it smooth before it sets. Plan to do one wall surface in a day.
Now smear the plaster on the wall [D]. I like to start in the bottom left corner and work upward as far as I can reach, laying the plaster on thin. Don’t worry yet about uneven results or marks from the trowel—these lines will disappear with successive trowelings. Once you’ve covered the reachable surface, go over it one more time quickly, then place your plank onto supports so you can access the top of the wall and continue applying plaster from left to right.
Cracks that haven’t pulled away from the backer lathe simply need bridging with nylon mesh; these walls will get two coats of plaster. As you place the plaster on the wall, embed the nylon mesh into the first layer of plaster [E], covering the entire wall and overlapping cracks for insurance [F]. The mesh is a reinforcing material that’s very durable and easy to incorporate in your skim coat, yet it cuts easily with a pair of scissors. You’ll need to overplaster the mesh coat after it sets to cover it completely.
As the plaster begins to harden and you continue working it, you’ll notice that trowel marks will gradually be erased (the plaster will start to feel soapy as it sets). You’ll need to make three to four passes over the wall, troweling fresh putty and pressing it down to consolidate, until the wall is smooth. Mix the second coat of putty soon after applying the first; the first coat needs to remain moist enough to adhere well to your second coat. The troweling procedure is the same, but without the addition of mesh it’s a little easier.
As you gain experience, you can experiment with the surface finish. For a high polish, sprinkle water onto the smooth, setting plaster and work it with the steel trowel until the finish is glossy. If you prefer a matte finish, stop working the surface after you’ve removed all trowel marks. If you’ve taped corners where surfaces abut, be sure to pull masking tape before the new application sets up, which will give you a nice, clean edge.
Jacob Arndt, principal of Northwestern Masonry & Stone Co. in Lake Mills, Wisconsin, has specialized in historic restoration masonry for three decades.