I have a brick wall that has had water drip down it for some time. How can I remove it? Is painting over it the only way?
Stiff nylon brush, spray bottle with vinegar, water hose, elbow grease.
Spray vinegar liberally over stains and start scrubbing hard. Rinse with water. Repeat.
Next level up for more stubborn stains would be to use diluted muriatic acid. 1 part acid to 10 part water. Dip brush into solution, scrub, let acid sit on brick for 10 minutes, rinse with water.
To be extra careful, you can then sprinkle baking soda on brick to fully neutralize any left over acid and then rinse again with water.
Protect any plants below with plastic sheets so runoff doesn’t get on them.
Stiff brush, strong vinegar if you [& the neighbours] can stand the smell, or descaler & detergent if you can't. It looks like hard water runoff, but it's hard to tell. Spray a small patch with regular kitchen or bathroom descaler & see if it fizzes. The darker parts might respond to a detergent.
For a lot less elbow grease, rent a Kärcher or similar - high-pressure hose system - advantage; cleaner & you can likely reach it all without a ladder. You'll be amazed at how deep one of those sprays can dig. I use one here for path cleaning after a filthy London winter, you can literally see the clean stripe of each pass. Downside - water everywhere.
Alternatively, rent a needle gun, e.g. https://brandonhirestation.com/tool-hire/concrete/concrete-scabbler-hire/110v-needle-gun $£€ 25 a day. That's only a one-day job.
Upside: it will be thorough. Downside - dust everywhere.
You'll need steps or a ladder & a 110v industrial transformer [probably from the same hire shop] You might find a 220v version, but 110v is much more likely as it's normally used on building sites etc where they always have to use 110v.
Eye protection essential for all three methods - the Kärcher & needle gun will probably get it so clean you will have to do that entire section of wall.