How to repair plaster walls over concrete?

The article aims to answer the question “How to repair the plaster walls over concrete?”. It will also discuss what plastering of concrete is and what considerations you need to take when you want to plaster the walls over concrete. 

How to repair plaster walls over concrete?

  • Sand the surface with a medium-grit sandpaper to remove any remaining plaster. Use a fresh towel to thoroughly wipe the spot.
  • This Old House advises using patching plaster to fill up the gap. Place a strip of fiberglass tape over the crack and patching plaster if the crack is more than 1/4-inch wide. Let the patching plaster dry completely before continuing.
  • Feather out the second layer of repairing plaster along the wall before applying the third. Apply a third layer of patching plaster if you used tape or if it is required. After the patching plaster has dried fully, use a fine-grit sandpaper to smooth off the surface.

What do I need to repair plaster walls over concrete?

Sandpaper

Trowel

Scrapper

How to repair holes in plaster walls over concrete?

  • Pull free loose plaster and clean the area clean with a cloth. Mix a ready-mix plaster foundation with water in a container, following the makers guidelines.
  • Fill the hole with a foundation layer of plaster using a medium-width trowel, recommends Dave’s DIY Tips. Scrape the plaster compound so that it rests 1/4 inch below the final surface of the plaster. Allow the plaster to set but not dry.
  • Apply a second layer of plaster using a trowel. If the hole is more than 4 inches, scrape the plaster out until it’s just below the current surface of the wall. 
  • Otherwise, smooth the plaster with the straight end of the trowel until it’s flat with the rest of the wall and smooth in appearance.
  • Apply a third layer of plaster if required after the previous coat has set but not dried. Ease the plaster until it’s smooth against the rest of the wall with a trowel. Once the plaster has cured fully, softly sand with a fine-grit sandpaper if the plaster didn’t finish totally smooth.

How to plaster concrete walls?

  • The concrete wall must be prepared. It’s necessary to remove all of the paint off a painted wall if it’s flaking or peeling. Sandblasting the wall is the quickest and most straightforward technique to do this.
  • If you don’t want to sandblast the painted concrete wall, use metal lath. Attach the lath to the wall using 7/8-inch concrete screws and washers spaced 12 inches apart.
  • Cracks should be filled, and mold and mildew should be treated by eliminating the mold and removing any stains it leaves behind. Use a wire brush to remove loose mortar from the joints.
  • When the paint has been well adhered to the wall, a bonding agent is not necessary. Use a spray or a brush to apply the chemical to the whole surface. Follow the directions provided by the product’s maker.
  • To finish the concrete wall, use a dash, slush, or slurry coat. Add enough water to make a runny paste out of 1 part Portland cement, 1 to 2 parts masonry sand, plus a few other dry ingredients. 
  • Dash the whole wall with a brush to a thickness of approximately 1/8-inch. Allow a day or two for the first coat to air dry completely before putting it on again. To make a base coat, the terms “dash,” “slush,” and “slurry” are all used interchangeably.
  • Use a slurry that is almost liquid in order to get a flat finish.
  • Make sure you follow the manufacturer’s directions while preparing the plaster. In order to get the best results, use a trowel to apply a base layer that is between 1/4- and 3/8-inches thick. 
  • For a flat wall, smooth out the plaster. For a textured effect, leave the wall as it is. Wait for the primer to dry.
  • Use the same method and mix of plaster to apply a second coat of plaster over the first layer.

What is meant by plastering of concrete?

During Plastering of concrete, in order to give a concrete surface a finished look, lean mortar is applied to it. Concrete surfaces can be plastered in a variety of ways, including dense, low-weight, and mixed concrete.

The surface of the concrete to be plastered must be free of flaking old plaster, peeling paint, and other contaminants such as oil and dust before the plastering work can begin.

The plaster will not stick to a smooth concrete wall. A slurry is applied to the concrete surface in order to achieve this. One part cement to one-and-a-half parts coarse sand is mixed with the amount of water necessary to achieve a runny consistency in a slurry or dash.

To achieve the required plaster consistency, a combination of fine aggregate (sand), cement, and water is stirred together. Using the plaster prepared in this manner must be done within two hours of its preparation.

A steel trowel is used to apply a base coat of plaster over the dash layer. The layer’s thickness should be between 10 and 15 millimeters. Make sure it’s level and uniform before applying with small strokes.

The thickness of the plaster should not exceed 10 to 15 mm if the plastering is done in one layer. Be sure to use a 5 to 10 mm top coat for the best results.

Immediately after the plaster has hardened and set, the curing process must begin and last for seven days. You can paint or wallpaper the plastered concrete walls once they’ve dried.

If Thistle Bond-it or another bonding agent is used, the undercoat shouldn’t be thicker than 11 mm on the walls and 8 mm on the soffits. Over the undercoat, the finish coat should be 2mm thick.

Applying the plaster over expanded metal lathing fixed mechanically to the concrete is necessary in areas where thicker layers are expected.

How to plaster dense concrete?

  • Plastering does not cling to the thick concrete’s surface because of its low porosity and smoothness. 
  • Due to the low porosity, lack of suction, and the absence of a mechanical key on solid concrete backgrounds
  • By mechanically scabbling or shot blasting the concrete’s surface, any contaminants may be removed and a smooth surface can be roughened.
  • Such a treatment may assist to enhance the plaster’s adherence, but it will not increase surface absorption and may not be enough to avoid failure alone.
  • Before plastering concrete, it should be allowed to cure for a sufficient amount of time after the shuttering has been hit. If there is any free water on the surface of the concrete, no plastering should be done.
  • Most new construction sites, on the other hand, do not have the time or resources to wait until the concrete has dried to the touch before proceeding, especially for structures that are quite thick.
  • Expanded metal lathing plugged and fastened into the concrete is probably the safest method of applying plaster. A drawback is that it might be pricey.

What considerations are required while plastering walls over concrete?

  • Concrete should be avoided as much as possible during plastering.
  • Plastered sections should ideally be covered with concrete that contains flint aggregate.
  • Rather of using mold oils, dab on a little amount of a chemical releasing agent.
  • It is imperative that the concrete surface be thoroughly inspected for contaminants and that corrective measures are performed.
  • Apply a special bonding agent, such as Thistle Bond-it, to hard concrete surfaces to remove surface dust. Use GypPrime on absorbent backgrounds to get the best results.
  • Plaster should be applied firmly with an eye toward checking for hollowness when the plaster cures and the structure is heated for the first time thereafter.
  • Avoid bringing the temperature of the building up quickly; instead, go slow and steady.

Conclusion

Plaster walls have a poor image, but they’re worth saving since they’re long-lasting, sturdy, fire-resistant, and sound-dampening. Prior to the 1950s, plaster was a common wall covering material, especially over lath and concrete. 

It’s better to fix the plaster rather than replace it if your walls have cracks, dents, or holes in them. For a smooth, clean surface once again, you might use repairing plaster over concrete rather than plastering over lathe.

Frequently asked questions (FAQS): How to repair plaster walls over concrete?

How to repair plaster walls over concrete?

Sand the surface with a medium-grit sandpaper to remove any remaining plaster. Use a fresh towel to thoroughly wipe the spot.

This Old House advises using patching plaster to fill up the gap. Place a strip of fiberglass tape over the crack and patching plaster if the crack is more than 1/4-inch wide. Let the patching plaster dry completely before continuing.

Feather out the second layer of repairing plaster along the wall before applying the third. Apply a third layer of patching plaster if you used tape or if it is required. After the patching plaster has dried fully, use a fine-grit sandpaper to smooth off the surface.

What is meant by plastering of concrete?

During Plastering of concrete, in order to give a concrete surface a finished look, lean mortar is applied to it. Concrete surfaces can be plastered in a variety of ways, including dense, low-weight, and mixed concrete.

The surface of the concrete to be plastered must be free of flaking old plaster, peeling paint, and other contaminants such as oil and dust before the plastering work can begin.

Bibliography

Plastering concrete surfaces. The constructor. Retrieved from: https://theconstructor.org/concrete/plastering-concrete/963/

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