4 Steps For Painting Cinder Block

Painting cinder block is not the same as painting wood siding. And there are unique considerations depending on whether you are painting new cinder block or an older house that has accumulated multiple coats of various types of paints over the years.

Cinder Blocks vs Concrete Blocks

Despite what most people think these two are not the same thing. Cinder blocks are rarely used in construction today though they were the predominant masonry unit (besides brick) until the late 1950s and early 1960s.

In the late 19th-century coal-burning factories dominated the industrial landscape of the US. All that coal produced huge quantities of cinders as waste until in 1900, Harmon Palmer devised a way to combine those wasted cinders with cement to form a “cinder block”.

These blocks were lightweight and structurally very strong, but in the 1950s as coal production decreased and new building codes called for even stronger masonry units the transition from cinder blocks to more modern concrete masonry units (CMUs) began in earnest.

Concrete blocks made with concrete and aggregates such as sand and gravel instead of cinders were denser, stronger, and less porous than traditional cinder blocks, making them more suitable for a wide range of structural applications in today’s construction industry. This transition has led to CMUs largely replacing cinder blocks, although the term “cinder block” remains in common use to refer to both types of blocks.

Preparation and Tools

Preparation for painting will depends on the which type of block you are talking about as cinder blocks are far more porous than their cement block cousins, though both are far more permeable than wood siding.

Due to their age, these blocks may have unique textural characteristics and potential structural weaknesses. Begin with a thorough inspection to identify any cracks, moisture issues, or efflorescence (salt deposits), which are common in older masonry.

Tools Needed:

• Stiff-bristled brush or a pressure washer for cleaning
• Chisel and hammer for removing loose material
• Masonry sealant for filling cracks

Cleaning Cinder Blocks

Cleaning is the first step in the preparation process. Use a low-pressure setting on the pressure washer (around 1500 to 2000 psi) to avoid damaging the blocks. In addition to removing any dirt you should use products like D2 or other mildew and algae killers to make sure the surface is completely clear of any growth.

Scrub with a stuff bristle brush to remove any stubborn stains or growth and rinse clean thoroughly so there is soap or cleaner residue.

Once clean, repair any structural issues with masonry sealant, carefully following the manufacturer’s instructions to maintain historical integrity.

#1 Use Block Filler For Unpainted Block

New or unpainted cinder is extremely porous like I mentioned earlier. That’s not a problem per se, but if you have unpainted cinder block or cement block then it’s imperative that you use a block filler to seal all those nooks and crannies that cover the surface of these.

Without a block filler the insides of these little nooks remains unsealed and can cause moisture issues and even bulk water passing into the walls which smells bigger troubles like mold and efflorescence.

What is block filler? Think of it as a very thick primer. It’s best applied with a thick nap roller (3/4” to 1” nap is best) so that the block filler gets into all the crevices. Property applied block filler will actually make the surface of the cinder block look much smoother than it is when unpainted.

Think of it like spackling over the surface to even things out. Once the block filler has had enough time to dry according to the manufacturer’s specs then you’re ready to apply the finish paint. You don’t need to apply a masonry sealer of block filler.

#2 Use Masonry Sealer for Painted Block

If you have previously painted wall then you have a much easier process when painting cinder block. Once the surface is clean, apply a masonry sealer like Seal-Krete or Loxon to the surface using a 1/2” nap roller or you can spray and back-roll the wall.

I do not recommend spraying only on masonry because of the porosity. Spraying alone without back-brushing or back-rolling doesn’t push the primer down into the crevices which results in poor coverage and a shorter life to the paint job.

These masonry sealers go on looking like thinned down skim milk and dry almost clear so pay close attention to make sure you are covering all areas. They create a strong, breathable bond to previously painted surfaces, are able to tolerate the high pHs common in cinder blocks and are water clean up making them easy to use.

#3 Use a Masonry Specific Caulk

There are almost always cracks or gaps in older walls so prior to painting cinder blocks it’s important to fill those gaps with a compatible caulk. Sherwin Williams Loxon line also makes a great caulk that in my personal experience works great for this purpose.

You can use other standard caulks, but personally I prefer a caulk specifically design for masonry because of the unique issues like high pH and high breathability needed for cinder blocks and concrete blocks. So stay from masonry caulks at your own risk.

#4 Use a Breathable Paint

When choosing paint, opt for breathable masonry paint that allows moisture to escape and prevents it from being trapped inside the blocks—a critical step to avoid further structural damage.

Lime-based paints or mineral paints are often recommended for their compatibility with historic masonry, but for the types of cinder blocks and concrete blocks found in mid-century and later homes those are not necessary.

I preference for me is again the Loxon line at Sherwin Williams. The Loxon Self-Cleaning Acrylic Coating is a relatively breathable option that work well for exterior masonry like cinder blocks.

I would advise anyone to stay away from elastomeric paints especially for any masonry like bricks or cinder blocks because it creates a virtually unbreathable layer that may look beautiful, but causes long term issues because it prevents the masonry wall from breathing.

Troubleshooting Common Issues

One common issue is paint peeling or blistering, often caused by moisture trapped within the cinder block. To mitigate this, ensure that any sealing and priming processes are thoroughly dried and cured before applying the topcoat.

Another frequent problem is color mismatch, especially when trying to blend new paint with existing structures. Always test paint colors in a small, inconspicuous area to ensure compatibility.

Other than that painting cinder block is relatively straight forward if you follow these four steps above. With the right approach, you can paint with confidence whether it’s a historic home or new addition. What masonry paints and sealers do you prefer if you’re not familiar with the products I mentioned that I use? I’d love to hear your thoughts in the comments below.