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Ask the Builder: Clean surface before patching concrete

DEAR TIM: I live in a house with a concrete block foundation. I think the outside of the block has decorative stucco on it because the exterior of the block is not smooth. Some of the exterior is crumbling, and I wonder if my foundation is going to fail. Can the damage be repaired? How do you get the stucco to stick onto the block and not fall off? Each time I’ve tried to do the job, the repair material falls to the ground. Help! – Lynn L., Mount Airy, Maryland

DEAR LYNN: If all of the concrete blocks have a similar-looking exterior pattern, I would say the blocks were made that way and that it’s not a stucco coating. Not long after concrete block was first brought to market, manufacturers figured out that they could make the outside of it look fancy, as if it were cut pieces of limestone or even granite.

You can look at the inside of your foundation and examine the condition of the block on the side that faces your basement or crawlspace. Look for similar crumbling or cracks. If the inside surface of the block looks good, then I feel confident your problem is just a cosmetic defect in the block. The good news is that this is very easy to repair.

Let’s discuss why your previous repair attempts failed. When you decide to repair the vertical face of stucco, concrete block or even concrete, the surface needs to be clean, dust-free and structurally sound. You should remove any loose, crumbling material by scraping with a stick. Remove it to a depth that, in order to remove more material, you would need a chisel and hammer.

The repair material you use must have enough cement and/or lime in it to make it sticky. The Portland cement and lime you’ll sometimes find in mortar mixes or bagged concrete is the primary ingredient that makes the repair material bite or stick onto something else much like paint, glue or epoxy adheres to surfaces. If you don’t have enough cement or lime in your repair mix, it won’t stick well.

Your mix may have been too dry and the concrete block may consequently have sucked too much moisture from the mix too quickly. This can also cause the repair material to be less strong than it could be.

I prefer to make my own repair mix using just medium clean sand and pure Portland cement. Although it’s nice to use hydrated lime in the mix, it might be hard for you to locate it. Portland cement on its own, when mixed with sand, will create a repair material that can last hundreds of years.

I prepare the concrete block by removing all loose material. Brush the block with a stiff brush. Washing the concrete block is also a good idea, flooding it with water and scrubbing the area at the same time. You’ll want the block to be slightly damp when you start the repair, so washing it will not hurt the repair process.

The repair material should be mixed no less than two parts sand to one part Portland cement. For extreme strength and stickiness, you can mix one part sand to one part Portland cement. You add enough water to make the repair mixture resemble whipped mashed potatoes or warm cake icing. This means the material is wet, has body and will easily hold onto the side of a trowel that’s held in a vertical position.

If the material droops, falls off the trowel or resembles a thick gravy that sloshes around in the bucket, you added too much water. Add more sand and cement in the proper amounts, stir and make the material thicker.

The magic trick most don’t know about is a bonding layer of cement paint. Cement paint was used by masons of old to help new stucco material bond to old stucco, concrete or concrete block. You make cement paint by just mixing Portland cement powder with clean cold water. Mix it to the consistency of regular paint.

With the cement block clean and slightly damp, take an old paint brush and paint the area to be repaired with this cement paint. Brush it on as you would paint, but not too thick. Only brush on as much cement paint as you can cover with the stucco repair mix in five or 10 minutes. You NEVER want the cement paint to dry on the concrete block so avoid working with this in the direct sun, on windy days or whenever conditions are such that it would dry rapidly.

Apply the stucco repair mix over the cement paint. Make sure it’s no thicker than 1/2 inch. If the area to be repaired is deeper or thicker than that, you may have to apply two coats of stucco. If you need to do this, be sure the first coat is left rough – resembling a miniature mountainous area – so the final or next coat has plenty of surface area to bite into. Wait at least 24 hours to add a second coat.

To create a smooth texture on the repair, use putty knives, or other small metal hand tools that will allow you to recreate the look of surrounding concrete block. Don’t try to texture the repair material immediately, as it may take 10 or 15 minutes for the stucco material to stiffen up allowing you to sculpt it with tools.

Need an answer? All of Tim Carter’s past columns are archived at You can also watch hundreds of videos, download Quick Start Guides and more, all for free. Have a question for Tim? Just click the Ask Tim link on any page of the website.


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