How To Paint Masonry

Real-world tips for dealing with brick, block and stone

From the perspective of your clients, painting is pretty simple. Wet, coloured stuff goes on buildings, spaces and rooms, making everything look better. That’s pretty simple and it’s all most non-painters want to know. But as a painting professional with a brush and roller in your hand, you need to know the tricks of the trade to make the magic happen. And this means understanding the challenges, chemistries, procedures and product choices that apply to the different kinds of surfaces you’ll encounter. Masonry is a case in point. You probably won’t need to paint these surfaces often, but when you do you need to get it right.

Painting Brick

Brick is one of the most porous surfaces you’ll paint and this affects the way you prepare brick and coat it. On the one hand the porous surface of brick is good because it gives paint something to grab onto. On the downside this same porosity is potentially dangerous because it can trap moisture or allow moisture migration that’ll lead to premature paint failure. Have you been asked to paint a newly-built brick wall? You won’t get requests like this often, but beware. It can take a year for excess moisture from the mortar to leave the brick. Your client won’t want to hear they need to wait so long, but better they be disappointed and call someone else than you have to deal with paint that’s coming off in sheets because moisture is migrating out of the brick.

Dangers of Painting Exterior Brick
Exterior brick can last for centuries, but only if freezing moisture doesn’t break it apart. If damp brick goes into winter weather with temperatures below freezing, this moisture will solidify, expand internally and break flakes off the brick. As this process continues it can completely destroy bricks. This is called spalling and these days modern brick is fired hot enough that moisture absorption isn’t normally a problem if left bare. Bricks are also constantly tested to make sure they don’t absorb damaging amounts of moisture and that they can dry out quickly.

The thing is, painting exterior brick can change all this. In most locations on a wall, a coat of ordinary paint will keep moisture out. Trouble is, if that paint film is incomplete or damaged, liquid moisture will enter the brick through the flaw and stay there. It’s virtually guaranteed. Impervious paint prevents drying and can lead to brick failure in time. This dynamic is why many brick manufacturers warn against painting exterior brick in a climate that gets winter. Explain this danger to clients to protect yourself against liability if they do decide to go ahead and paint brick anyway. If you do paint exterior brick in a cold climate, be sure to use breathable paint. Google “breathable paint” to find the options. For an even simpler and breathable choice, see Traditional Lime Paint for Brick & Stone.

The next time you get a request to paint brick, explain that it’s a once-and-for-all move. Short of sand-blasting, it’s impractical to return brick to its original, unpainted state. This sounds obvious enough to you as a pro, but don’t forget how much explaining non-painters need some times.

The good news in all this is that 100% acrylic exterior latex primer and paint works well for all interior brick. The thing is, you can’t just slap a couple of coats on brick and leave. At a minimum you’ll need to clean the brick and allow time for it to dry before painting. If you’re working outdoors, pressure washing on its own is good, but wetting, brushing, then pressure washing as a final step is even better because it removes dirt more thoroughly, plus any loose particles of mortar sand or brick. Of course you’ll need to skip the pressure washing indoors, but use TSP to make up for this as part of washing only by hand. Either way, leave the surface for three good drying days to allow it to dry completely after a thorough cleaning.

Got a request to paint exterior brick? See “Dangers of Painting Exterior Brick” for key info.

The next time you get a request to paint brick, explain that it’s a once-and-for-all move. Short of sandblasting, it’s impractical to return brick to its original, unpainted state.

Most of the brick you’ll be asked to paint will be old, and this could mean broken bricks or loose mortar joints. You can’t expect reliable painted results on a shifty surface, so remove loose bits of mortar as you clean. Is your client reluctant to spring for a full repainting job? That’s typical, but if failed mortar joint areas are less than 1” long and 3/4” deep you can fill them with paintable caulking. The best is polyurethane. It’s not as common as other types, but it’s worth it. Polyurethane caulking is more flexible than most: it sticks like crazy, and it’s completely paintable. Squeeze some into gaps, then dip your finger in a solution of 70% water and 30% dishwashing liquid to smooth out the results.

Even if the caulking is close to the same colour as the mortar, it’ll still stand out visually at this stage. Things will look much better after the paint goes on. Another nice thing about polyurethane caulking is that a little moisture actually helps it cure better than dry conditions. Do your caulking work after the initial wash water has dried superficially, but before the wall has dried out enough for painting. This way both the wall and the caulking are drying at the same time.

Staining Brick
Paint isn’t the only way to change the colour of brick. Stain is an option, too. Stain doesn’t create a sealed or semi-sealed surface like paint can, so stain doesn’t pose a risk for holding moisture within the brick. Brick stains can be used both outdoors and inside. If you’ve never used any before, practice on the brick with water. Brick stain and water are the same consistency so the experience you gain avoiding runs and loading your brush will be the same.
Neatness and an even application are necessary for even results, so practice is worthwhile.  Most brick staining jobs look best when the brick face only is coated. If you’re leaving the mortar joints unstained, use a high-quality latex brush for the precision you need. One little-known application for brick stain is when matching the new brick of an addition or repair to the existing brick.

The thing about painting brick is that it’s not an all-or-nothing venture. Results are more like points on a gradient. Undiluted primer and paint straight from the can will hide all the colour of bricks and some of the texture. And the more coats you add the more texture will be hidden. Does your client think they might like some of the original brick colour to show through, but not too much? Paint washing is what you need. Some people call this brick wash and it’s nothing more than brushing diluted latex paint onto the wall. Brush it into the mortar joints and on the face of a couple of bricks, brush out any excess, then move on to the next area. How much water should you add to the paint? That depends on the look your client wants and the porosity of the bricks. Moving from a more dilute mix to a more opaque one is the way to home in on the type of water-to-paint mix ratio. Start with a light mix, then add more paint, stir it completely, then try again if you want something more opaque. You can always go heavier, but not lighter.  Start with 30% paint and 70% water then go thicker from there. Many painters find that a 50/50 blend of paint and tap water works fine.

Painting Concrete Block

The surface of concrete block is usually smoother than brick, but block is also used differently than brick in ways that affect painting. Block is almost always a structural element, including basement foundation walls, structural exterior walls, retaining walls and as dividing walls between properties.  The same requirements for cleaning brick before painting applies to blocks, but there are differences you need to know about.

When block is used below ground as part of a basement, there’s a very good chance inward moisture migration is happening, even if things seem dry. If you see white, fluffy formations on the block surface, you can’t simply clean it off and paint. This white stuff is called efflorescence and it’s a sure sign that moisture is making its way into the walls. Not enough moisture to be visible, but more than enough moisture to cause paint failure.

As small amounts of miner-al-laden moisture make their way through masonry from the outside, the moisture dries as it comes in contact with indoor air, leaving the minerals it carries high and dry. This inward flow of moisture isn’t large, but it’s constant, resulting in the formation and growth of those white fluffy deposits you see.  It’s crucial you understand this as a painter because it means you need to waterproof the wall to stop the migration before reliable painting can happen. Efflorescence is some-thing you must never ignore.

Two waterproofing options with excellent track records are Drylok and Xypex. You can apply them to any masonry surface as long as no flowing water is moving through the wall. Clean the blocks, let them dry, then apply the waterproof coating before paint. Both Drylok and Xypex can only be applied to bare masonry. They need to soak into the pores of the masonry to form a waterproof layer below the surface. In the case of Xypex, this product has been independently tested to withstand 175 psi of hydrostatic pressure. That’s way more than anything you’ll encounter in a basement.

So what can you do for a client with a block wall where existing paint is failing? You can prep the surface and make it look good for a while, but you also need to explain that the fresh paint you’ve applied won’t last. Submit that in writing and get a sign-off before you paint over a peeling block wall surface. If the flaking of the old paint is extensive enough you could use a bushing head in a small rotary hammer to knock off the flaking paint and reveal fresh masonry. Anywhere you can get rid of the old paint can be water-proofed permanently.

Although blocks are a masonry surface, there’s no need to use a breathable paint on the inside of a basement block wall that has been waterproofed. The interior side of foundation walls aren’t subject to freezing and thawing that causes spalling, so 100% acrylic primer and latex work fine.

You probably won’t have to paint brick, block or stone very often, but when you do it’s wise to under-stand the challenges. Every time you tackle a paint job you’re creating a possible headache for your-self if your work doesn’t last like it should. Minimizing your risk is one of the biggest reasons to get good at all the different masonry situations that come your way.

Traditional Lime Paint for Stone
Have you ever seen those brilliant white old stone buildings? Or perhaps you’ve visited the inside of a traditional barn with a stone foundation that was also white? Chances are this isn’t paint, but lime whitewash. It’s the olden-days coating used to create this effect, and like many time-tested methods, there’s good sense in it. Lime whitewash is made with hydrated lime and fine salt mixed in a ratio of 5 parts lime to 1 part salt. Add enough water to create a slurry that’s like thin cake batter. Clean the wall, then brush the whitewash on. Choose a cloudy day if you can since you don’t want the whitewash to dry out quickly. It’s economical, long lasting and it allows moisture to pass through it easily. You can also add powdered pigments to lime whitewash for colour.

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