Muriatic acid is a chemical primarily used in swimming pool treatment. It can also be used for removing paints and other substances from surfaces like bricks, concrete and stone that can't be cleaned easily. This is, however, a very corrosive and dangerous material. You should only use this if you cannot remove the paint in any other way. Use extreme caution when handling this or any other dangerous materials, and make sure you know all local/federal laws for disposing of the acid.
Things You'll Need
Cover all areas surrounding the paint stains with tarp or similar sheets of thick rubber or plastic. At the same time, wear long-sleeved clothing and gloves that will fully protect all body parts and add protective eyewear.
Mix the acid with water, pouring the acid carefully into the water from a glass graduated container; you need four times as much water as muriatic acid.
Dip a stiff nylon brush into the water/acid solution. Let all excess water drip from the brush over the bucket before moving it toward the stains.
Scrub the stains out of the surface with the acid-soaked brush.
Rinse off the scrubbed area quickly after scrubbing using a rag soaked in fresh water.
Transfer the water/acid mixture into a disposable container that you can seal and carefully transport it to a hazardous waste facility that can dispose of it. Wash and rinse out all other containers and objects that used the acid.
Baking soda will neutralize this acid, but it can release dangerous gas in the process. Use it to clean spilled acid only if absolutely necessary.