Concrete walls present a unique problem when it comes to acid stain. Most acid stains available for concrete are the consistency of water. If you try to apply this acid stain to your wall, it won't adhere, but will run down the wall and all over your floor. However, there is a solution if you wish to add depth and color to your concrete wall: gel acid stain.
Prep the wall. If your wall has been previously waterproofed, you will not be able to use acid stain. If your concrete wall has any sort of residue from paint, varnish or adhesives, use a paint or adhesive stripper made to remove the residue in question before you apply the stain.
Cover any surrounding surfaces that you want to protect.
Put some of the gel concrete acid stain in a paint tray.
Use a foam brush or paint pad to apply the stain. If you are working with one color, apply an even coat over the entire surface. If you are working with two colors, depending on the look you want, you can overlap in random patterns to create a "marble-like" appearance.
Allow the gel concrete acid stain to cure on the wall according to the package directions. Curing times range from two to eight hours.
Neutralize the concrete acid stain using a pump sprayer to spray the wall with a mixture of 1 to 2 tbsp. baking soda per 1 gal. of water. Use large sponges to sponge up any runs down the wall.
Clean the entire wall with clean water to remove any remaining residue. Use a nylon scrub brush to remove stubborn residue.
Let the wall dry completely. This may take up to 24 hours, depending on your climate.
Use a wide paint pad to apply two coats of the sealer. Let each coat dry thoroughly according to the manufacturer's directions before applying the next coat.