How to Patch Plaster Walls in 10 Steps

Before beginning a repair project like this, take steps to protect your home (and yourself). Lay down a drop cloth in your work area, secure it with painter’s tape, and wear goggles and a mask at the appropriate times. Also, be prepared to wait for possibly lengthy plaster drying periods.

Plaster walls can be decades or even centuries old. Because of this, any work you do you may only be able to do once. If, for example, you damage the underlying plaster, it can be nearly impossible to match it with a repair. Treat plaster repair with much more care than you might for drywall or Sheetrock. The saying “measure twice, cut once” is something to remember when working with plaster.

Gently remove large pieces of plaster with either a box cutter or hammer. If you absolutely need to, chisel until you have a smooth and even working surface. Any shards that remain will create inconsistencies in your wall if left behind and will make it difficult to blend the repaired section into the undamaged sections. Plaster is known to break off in unique ways, but you’ll want a surface that’s as uniform as possible before you begin repairs.  

Vacuum up as much dust and debris as possible from your wall and workstation, and then use a microfiber cloth to remove anything that still remains. Finally, use an all-purpose cleaner and rag to clean the area surrounding the hole.

Lath is the wooden frame that will be underneath the plaster and is easier to replace than repair. Plaster and wood are both materials that can last a long time but lath can be damaged from conventional force—like holes put through the wall—but can also be caused by water damage, fire, and humidity. 

Replacing and repairing lath is a significant job that often requires the removal of the plaster on top of it. Assuming support beams are in good condition, removing lath isn’t too complicated (it’s basically a demolition job). Remove the lath with a crowbar or hammer until there is none left. 

At this point, you would replace the lath with a wood of your choosing. Lath boards are widely available at home improvement stores, and they aren’t too expensive. Nail or screw the lath into the support beams, leaving enough space to plaster on top. A good gauge is using your little finger as a spacer (around one-third of an inch). 

If you discover the lath is metal, simply rip it out and replace it with the same material. It’s rare that metal lath becomes damaged, but it isn’t too expensive and will save you from having to remove plaster on top of the old metal lath.

You shouldn’t replace or repair plaster without reinforcing existing plaster, just as you wouldn’t build a house without a foundation. Drill holes in the existing plaster every 3 inches around the damaged area. You don’t have to be exact, but you do need to make sure there is enough space to create a secure bond.

Once the holes are drilled, fill them first with plaster conditioner and once that is dry, fill the holes with plaster adhesive. This ensures that the first coat of plaster you add will adhere properly. The last thing you want is plaster falling off your wall.

The scratch coat (sand, water, and cement) is the first coat you’ll apply to your lath. It may initially seem like you’re applying a lot of layers, but each one has a specific purpose. Scratch coats are applied so that the brown coat adheres properly to the lath while also securing the lath in place.  

A scratch gets its name from the “scratching” you do to the mixture once it’s applied. By running a handheld tool—that looks like a rake—through the mixture once it’s spread on the wall, a series of lines are created. If you have a ridged trowel, you can use the toothed side of it in the same way. The following coat that is applied will stick to those lines while on the other side, the scratch coat bonds to the lath. It’s vitally important to spray your path lightly with water so that the plaster properly adheres to it. 

Once the scratch coat is applied and has had adequate time to dry, it’s time to apply a brown coat. This second coat is usually the same mixture as the scratch coat and is applied so that the finishing coat has a uniform and flat surface to adhere to. 

You don’t necessarily have to be precise when you apply a brown coat. In fact, some people prefer a rougher plaster texture. If you are trying to finish the job with a smooth wall, you can save time by spreading your brown coat as flat as possible.

A brown coat is thicker than a scratch coat. A good thickness for a brown coat is 3/8 of an inch.  

The finishing coat is the last coat you apply when you’re repairing or replacing plaster. This last coat is made of joint compound and is spread very thinly over the brown coat. Using a trowel, spread the compound as evenly as possible. Let it dry, and repeat the process twice for a total of three finishing coats. This may seem redundant, but it’s necessary to achieve a sturdy wall with the right aesthetic.

Once your spackle or joint compound is dry, which can take 24 to 48 hours depending on the humidity levels in your home, sand it with a hand sander until it’s smooth. Repeat the previous step, if necessary. Be sure to take safety precautions, like wearing goggles and a face mask, to keep yourself safe from the dust as you sand.

"The most important step in repairing plaster or gypsum walls is sanding," Tschudi says, "because all that matters is the final look and a seamless patch will make the repair just disappear."

When the wall patch has fully dried, you’re free to do the fun part: paint! Since you should never paint over damaged plaster, this is a good opportunity to try out a new paint color or try that stylish accent wall you’ve been dreaming about.