How to repair plaster on concrete walls?

[skrundarlow]

My wife and I have bought an old 60s brick house. The internal walls are cement with as far as I can tell a painted plaster coating.

There are a number of places where the plaster is loose or cracked and needs to be repaired. In the first image you can see an area where we have chipped back a loose section to expose the concrete.

What approach could I take to repair this wall? If I sand back & prep the remaining undamaged plaster would it be feasible to patch areas like this with new plaster?

As a secondary question, the concrete walls have expansion joints that were filled with a lighter grout/cement that has cracked in parts. Refer to the second picture. 

The light is because the joint goes all the way through to the next room. 

How/what type of product would you recommend to fill the expansion joints before refinishing the wall?

[EricL]

Hello @skrundarlow 

Welcome to the Bunnings Workshop community. It's sensational to have you join us and thanks for sharing your question about repairing the plaster on your concrete walls.

To repair the areas of damaged plaster and expansion joints in your 1960s brick house, here’s an approach you can take using products readily available at the store:

Repairing Damaged Plaster:
 

1. Preparation: Start by removing all loose or cracked plaster around the damaged areas. Sand the edges of the remaining undamaged plaster to ensure a smooth transition with the patched areas. Clean the exposed concrete thoroughly to remove dust and debris, which will help the new plaster adhere properly.
    
2. Bonding Agent: Apply a concrete bonding agent (such as Bondall BondCrete) to the exposed concrete surface. This ensures that the new plaster adheres securely to the wall.
    
3. Patching: Use a pre-mixed patching compound like Selleys Spakfilla Rapid for smaller areas or Dunlop Multipurpose Acrylic Render for larger repairs. Apply the compound in thin layers using a wide trowel, feathering the edges to blend seamlessly with the existing plaster. Allow each layer to dry before applying the next to avoid shrinkage or cracking.
    
4. Finishing: Once the patched area has fully dried, sand it lightly to achieve a smooth finish, ready for priming and painting.

Filling Expansion Joints:
 

1. Clean the Joint: Remove any loose or cracked material from the expansion joint, ensuring it’s clean and free from debris.
    
2. Flexible Filler: For filling the joint, use a flexible polyurethane sealant like Sikaflex Construction+. This type of sealant is designed for expansion joints, as it remains flexible and accommodates movement.
    
3. Application: Use a caulking gun to apply the sealant into the joint, ensuring it’s filled completely. Smooth the surface with a caulking tool or your finger (dipped in soapy water) for a neat finish.
    
4. Optional Backing Rod: If the joint is very deep, insert a foam backing rod into the joint before applying the sealant to help fill the space and reduce the amount of sealant needed.

Once the repairs are completed, prime the patched areas and joints with a primer suitable for masonry or plaster, such as Dulux 1Step Prep, before applying your final coat of paint. This process will leave you with a smooth, durable surface ready for a fresh coat of paint and a restored appearance.

If you need further assistance, please let us know.

Eric