Ask the Authority - Plaster Repair

A crack opener is used to scrape out the loose plaster at the center of a crack and flare out the edges.

Q: The plaster in my house is riddled with cracks. Is it worth the trouble to repair it, or would it be wiser to go ahead and replace it?

A: Removing plaster and lath is a straightforward process, but it’s unbelievably messy. Depending on where you live, disposing of the debris can also be extremely costly.

And if you’re an old-house lover, you need to be aware that the drywall you install as a replacement will never match the solid, if imperfect, character of old plaster. So, if it’s just cracks you’re seeing, rather than large chunks of crumbling plaster, I’d recommend patching them.

Plaster washers and screws reattach loose plaster to framing.

To fashion a strong bond between the old plaster and the patching compound, it’s important to widen the crack and flare out the edges (photo). Plaster is brutal on sharp tools, so I use two different cutting implements for this process. First I use a standard utility knife, to separate the paint and the top layer of plaster. The blade dulls quickly, but a few passes across a pocket-sized whetstone restores enough of an edge to keep working. After the initial incision, I reach for a small hand tool called a crack opener (www.hydetools.com) to scrape out loose plaster and "v-out" out the edges of the crack. For small jobs, a bottle-opener or a painter’s 5-in-1 are equally effective, but the crack opener is made from high-carbon steel which holds a sharp edge much longer.

A spade bit is used to countersink plaster washers.

As long as the crack survives the scrape-out without crumbling into chunks, it can be filled and taped much the same as if it were a drywall joint. Before breaking out the mud, however, it’s important to verify that the plaster is firmly attached to the lath (the horizontal wooden strips which attach the plaster to the studs); otherwise, future cracking is inevitable.

I apply pressure with my hands at various locations along either side of the crack. If the wall feels the least bit spongy, I’ll use plaster washers and screws to secure the lath and plaster tightly to the framing. Plaster washers (www.charlesstsupply.com) are wafer-thin, perforated discs which are designed to reattach loose, but otherwise sound plaster to the framing without causing cracks. After locating the centers of the framing members, I drill 3/32-in. pilot holes a couple of inches away from each side of the crack and fasten the washers using 2-in. screws.

Setting compound (Durabond 90) is forced into the crack from both sides to ensure all voids are eliminated.

Although plaster washers are designed to flatten as the screw is driven home, they nevertheless, remain slightly proud of the wall surface. For most situations, you can achieve a nearly invisible patch by feathering out the repair a few inches wider than you would for a drywall seam.

If the repair is in a highly-visible location, I prefer to start with a flush surface, so I’ll countersink the washers using a1 1/8-in. spade bit. This procedure takes a bit of practice, however, because you need to bore through the paint layers but stop the bit as soon as it carves a circle in the finish coat of plaster; if you drill into the brown coat, the plaster won’t be strong enough to withstand the pressure from the washer, and you’ll have to start another hole.

After wiping away the excess joint compound, the crack is covered with fiberglass mesh tape, which is soon covered with a second layer of compound.

When all of the washers have been fastened, I vacuum the crack to remove any debris, then lightly mist the surface with water to prevent the dry plaster and lath from drawing moisture out of the patching compound before it’s had time to cure.

I use a setting-type joint compound like Durabond 90 to fill the gaps, mixing up only as much as I can apply in about an hour. After packing the joint with compound, I cover the seam with fiberglass mesh tape, and follow up with a second layer of compound. The advantage of using Durabond (over a traditional plaster) is that it sets up as hard as plaster but dries quickly, so if I’m in a hurry I can apply a second coat the same day. Durabond is almost unsandable, though, so I make sure to wipe the edges and overlaps clean.

I use all-purpose, ready-mix joint compound for the succeeding coats, and I apply these exactly the same as if I were finishing drywall.

Tom O’Brien is a veteran restoration carpenter who writes frequently about construction practices and old houses.