Plaster Repair Wall

Plaster Repair Wall | The white coat of finish plaster has delaminated from the coarse brown coat. If you’ve got intestinal fortitude, you might be able to rescue this wall. Photo Credit: Travis Boyer

DEAR TIM: My wife and I removed beadboard wainscoating in a house we just bought. In the process, we destroyed the wall.

The beadboard was glued on and when it came down so did a thin coat of something. I think it's plaster. Why are there two different materials and why did the wall fall apart? How do I repair it?

We were thinking of putting up walnut paneling over the mess to hide it. What would you do? Travis B., Holladay, UT

DEAR TRAVIS: Based on your photo, you're neck deep in a tough repair job. The walls in your house are indeed plaster. It's a superb wall and ceiling finish material that's slowly marching down the timeline from history, to legend to myth. Many ancient buildings have plaster walls and just about every church I know of, including the Sistine Chapel has plaster finishes. Years ago, plaster was put in every house. Now I'd estimate that less than 0.0001% of new houses have plaster.

What is a plaster wall?

Plaster can last hundreds, even thousands, of years because it's rock. Plaster is simply gypsum rock that's mined from the ground. The rock is crushed and heated in a mill to drive out the natural water hidden in the mineral. This chemically unstable gypsum is then ground up and put into bags. When you mix it at a job site with water, you start a chemical reaction that transforms the powder back into solid rock as tiny crystals begin to form, interlock and harden.

This is why it's so hard to pound a nail into a well-built plaster wall. You're trying to drive a soft-steel nail into rock! Plaster is also wonderful because it won't burn. It's easy to paint and it's very durable.

How are traditional plaster walls made?

Traditional plaster walls were always two or three-coat systems. A base coat of plaster was put over wood or metal lath. About eighty years ago, plaster lath was invented. These were small sheets of drywall that measured 16 by 48 inches.

This base coat plaster had sand added to it much like you have larger stones in concrete. The sand added volume and strength. The final thin coat was lime putty mixed with very fine gypsum plaster. The consistency of this material was much like cake icing. Skilled master craftsmen would trowel this on over the rougher brown or base coat. As the crystals grow, the troweling action would transform this white-coat plaster into a hard surface as smooth as glass.

Your wall fell apart because of the adhesive chain. When you have certain things glued to one another in layers, the weakest bond is what falls apart when you subject the layers to tension or pulling.

In your case, the top or final coat of plaster came off where it contacted the rougher base coat. But on other places of the wall I can see where the bond was weakest within the base coat of plaster.

How can you repair a damaged plaster wall?

You can repair this horrible mess if you possess some decent hand-eye coordination. The first step is to remove all the loose material by chipping and scrapping it away. You can buy base coat plaster called Structolite that can be used to fill in the deep areas where the original base coat is missing. You trowel on this new sandy plaster and rub it with a coarse rubber or sponge float.

It's surprisingly easy to apply and finish this Structolite. Start with a small amount and play with it. Only mix up enough that you can use in 15 minutes and might cover just a couple of square feet. Get the hang of working with it.

I'd not recommend you try to use traditional lime putty and plaster as that requires some training. You can buy dry setting joint compound that's the baby brother of real plaster. This gypsum product has a small amount of real plaster in it and when you mix it with water it starts the same chemical reaction. As the joint compound gets hard, you can smooth it with a trowel and wet sponge.

I don't know if I'd install the walnut paneling. Based on your photo the room seems small and there's not a great amount of natural light. The walnut paneling will make the room feel even smaller than it is.

Realize you'll have to strip all the baseboard from the room before you do the paneling. You'll also have to do that with all the window trim and any door trim. What do you intend to do with the painted window? It won't match the walnut paneling.

You'll also have to install a custom extension jamb at all windows and doors as the paneling will extend beyond the face of the existing plaster. This is not as easy as it seems and requires some good carpentry skills.

If it were me, I'd futz around repairing the damaged plaster and paint the room a nice bright color. You can also wallpaper the room if your plaster repair is a little bumpy. Select the right wallpaper and the bumps will be mostly hidden by the wall covering.

This column was so popular, Tim featured it as the Mystery Link in his September 13, 2015 AsktheBuilder Newsletter.

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