Acid Stain is not paint or a type of coating. It is a very affordable and unique way to give concrete an irregular, variegated or marble -like look. This process is a chemical reaction on concrete that causes a beautiful reaction of color. There are techniques that can be applied to both interior and exterior concrete. Other benefits of this staining is that it is very easy to maintain, and can often be done on your existing concrete, won't rust and is flood proof.