Most stains used in acid etching are a mix of water, hydrochloric acid and acid-soluble metallic salts. They work by chemically reacting with the concrete.
First, the acid etches the surface allowing the metallic salts to penetrate more deeply. The stain then reacts and the resultant colour becomes a permanent, sub-surface part of the concrete.
As a result, acid based chemical stains offer some key advantages over paint such as the fact that they won’t crack, chip, peel or fade.
As for the drawbacks? Well, these stains come in a fairly limited colour palette typically restricted to earthy tones such as tans, browns and terra cottas.
The colours are also unpredictable. The colour you see in the tin can be different to the colour you end up with because the staining relies on chemical reactions. Colours can also be blotchy in their intensity.
Bigger issues stem from the use of hydrochloric acid. Acid based stains must be applied by trained professionals familiar with COSHH working practices. As such, while product costs for acid based stains are comparable to those of paint, labour costs are invariably higher.
Acid based chemical stains also give off unpleasant odours which can linger and they are messy.
After their application, the treated surface must be neutralized with a mild alkaline detergent. After that, the surface must be washed with liberal amounts of clean water.
Acid based chemical stains are also unsuitable for detailed work as the colour tends to weep into surrounding areas.