I bought a brand new gas-powered pressure washer last fall. It came with a neat little row of colored plastic nozzles clicked into the handle. The instruction manual was essentially just a single sheet of paper with a few safety warnings. I decided to start by washing the old brick on the front of my house before applying a clear masonry sealer.
I grabbed the red nozzle because it looked like it would give me the most concentrated power. I pointed the wand at a patch of green moss growing near the foundation and pulled the trigger.
The red nozzle is not for cleaning. The red nozzle is essentially a water laser. It did not just remove the green moss. It instantly blasted a quarter-inch hole directly through the eighty-year-old mortar holding my bricks together. A shower of gray sand and pulverized concrete hit me in the face. I panicked and swept the wand sideways, carving a completely clean, deeply recessed line straight across three different bricks.
I had to spend the entire next weekend learning how to mix mortar in a plastic bucket and repoint the joints I had completely destroyed in four seconds.
If you are washing an older house, throw the red nozzle in the garbage immediately. Stick to the wider, fan-shaped green or white nozzles. They are more than strong enough to strip dirt and flaking paint without turning your exterior walls into expensive structural damage.