Home Improvement Tips: How to Stain Concrete with Acid

Concrete slab construction is cheap, durable and effective. With passive radiant heating, it’s also energy smart. Unfortunately, concrete slap floors are ugly. Nobody likes living in a house with gray concrete floors. Acid concrete stain is an excellent way to beautify your home.

By using a dilute mixture of hydrochloric or nitric acid, water, metal salts and other materials, you can apply an acid stain to a concrete floor as a home owner. Although called acid stains, the acid doesn’t actually color the concrete that much. It’s the metallic salts in an acidic, water-based solution reacting with hydrated lime (calcium hydroxide) in hardened concrete to yield insoluble, colored compounds that become a permanent part of the concrete. Several companies manufacture chemical stains that are variations of three basic color groups: black, brown, and blue-green. First, do some research on what mixtures of what materials give the best colors. Most staining processes leave the concrete with a warm, rich earthy hue.

Because concrete is very porous, acid stains tend to seep in – sort of like water into a sponge, which makes the application process take a bit of time. The acid wash is mopped onto the floor, much like a floor cleaner is, and dries fairly quickly. For solutions with silver chlorides in them as the metallic salts, you can vary the darkness of the pattern by shining lights on them as they dry. A little bit of experimentation will give you a lot of subtle patterns to work with.

When applying the acid concrete stain, watch out for fumes, and don’t wear anything in the staining process that you’re fond of – even diluted acids will do a number on shoes, socks and clothing. Once the acid stain has set in and dried, you need to put in a couple of layers of sealant to keep it from getting dulled by people walking on it and tracking dirt over it.

Some tricks to consider:

You can make a faux tile pattern by laying down masking tape on the cleaned concrete floor before applying the stain. Ã?¼” Quilter’s Tape is best for this. This will leave you with little strips of gray concrete in your staining pattern. Because you lay this down directly, you can adjust the geometry in ways you couldn’t do when laying tiles down.

Before you lay down the sealing coats, consider using stamps or a second coat of stain to paint patterns on the floor. With the right choice of metalized paints, you can get startling effects – sunbeams, “blonde” trims going through a deep earth toned floor, darker highlights, and more.

If you’re truly inspired, and have the time, consider taking a grouter and a template to incise patterns into a floor. As this will be quite time consuming, we recommend doing it in moderation. You can even apply paint in the grooves (or colored tile grouting mixtures and then paint) before applying the sealant, to give your floor an “inlaid look”.

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