Can I Apply Mortar Over Cinder Block Walls?

Cheryl recently came across a great deal on STO Powerflex medium and is planning to repair the stucco on her cinder block house. She’s wondering if she can apply mortar over the cinder blocks as a base before finishing with the STO product.

Question from Cheryl:
I have a cinder block house needing stucco repair. I recently acquired several buckets of STO Powerflex medium at almost no cost to me. My question is: Can I apply mortar over cinder blocks, and then STO Powerflex over the mortar? Thanks!

Answer: Hey there Cheryl! Yes, mortar can work as a base coat over the cinder blocks, but mixing up a base coat stucco material in bulk will work much better and be significantly more cost-effective. Here’s why...

Why Use Base Coat Stucco Instead of Mortar?

• Better Workability: Base coat stucco materials, made with plastic cement, sand, and water, are designed for stucco applications, making them easier to apply and work with, compared to mortar.
  
  
• Cost Savings: If you’re covering a large area, mixing your own base coat stucco will save you a lot of money while still providing excellent performance.

Steps to Repair Your Stucco...

1. Pressure wash the walls thoroughly to remove the loose paint and clean the surface up before doing anything.
   
   
2. Mix the Base Coat: Combine plastic cement and clean sand at a 1:3 ratio (1 part cement to 3 parts sand). Add water gradually until you reach a workable consistency.
   
   
3. Apply the Base Coat: Trowel the base coat stucco onto the cinder blocks, ensuring even coverage and keying it into the surface for a strong bond. Smooth out the layer and allow it to cure for a week or so.
   
   
4. Prime the Base Coat: Before applying the STO Powerflex finish, it’s highly recommended to prime the base coat. A primer seals the base coat, providing even suction, which ensures consistent texture and color in the finish coat. STO typically offers compatible primers for this purpose.
   
   
5. Apply STO Powerflex Finish: Once the primer has dried, apply the STO Powerflex finish coat according to the manufacturer’s instructions. This will provide a durable, attractive finish that protects your stucco.

Why Priming is Important

Priming the base coat ensures:

• Even Suction: This prevents the base coat from pulling moisture out of the STO finish unevenly, which could affect its appearance and performance.
  
  
• Consistent Color and Texture: Primer creates a uniform surface, reducing the chances of color or texture inconsistencies in the acrylic finish.

By using base coat stucco material instead of mortar, and taking the time to prime the surface, you’ll achieve a higher-quality repair that works seamlessly with your STO Powerflex finish. Let me know if you have any questions about the process!