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Painted exterior brick does not usually need sealing. Brick is a porous material, so you don’t want to apply a sealant that could trap moisture inside the bricks and cause erosion and structural damage. The risk of moisture-induced damage after sealing is higher in colder climates where trapped moisture can expand if it freezes. On the other hand, it makes sense to seal really old brick or kitchen backsplashes, where moisture from cooking can weaken the brick.

While painting can certainly change the look of a chimney, you should think twice about giving your brick chimney the old landlord special.

Painting brick is porous, meaning it’s full of tiny holes that can soak up moisture. Those holes also allow it to dry, though. So, when you paint it, you trap any moisture inside, providing an opportunity for mold, mildew, and water damage.

Paint also shows wear faster than naked brick. This means that once you paint a brick surface, you’ll have to keep painting it to maintain a clean look. A paint job is also harder to undo on brick because of its porous nature. Once you paint it, it’s hard to go back without some serious power washing—another added cost.

Most of the time, you don’t need to seal brickwork and can save that money for another area of home improvement. But if your brick is cracking or you live in an area with a lot of excess moisture (either in terms of rain or freezes), sealing can extend the lifetime of your home. But, make sure you use a breathable sealant, like siloxane, to allow the moisture to evaporate on its own.

In some cases, black stains on brick are caused by organic substances like algae and mold. However, it's more likely that the black spots you're noticing on your bricks come from carbon from vehicle exhaust or soot. Black stains can usually be removed by scrubbing the brick with a brush and masonry cleaner or power washing.