Acid etched finish
Acid etching is a method of removing laitance from the concrete surface to expose the colour and texture of the matrix beneath. Hydrochloric acid is used, either in diluted form or as a gel. The level of dilution of the acid, and the length of time that it is left in contact with the concrete before being washed, off control the depth of exposure. The process may be repeated to achieve a greater depth of etching. Care must be taken when acid etching vertical surfaces to avoid streaking. Ideally etching should be carried out three or four days after casting, when the concrete has achieved sufficient strength, but not to the extent that it is difficult for the acid to penetrate. After etching it may be necessary to make good minor blow-holes exposed by the removal of the laitance.
An alternative to acid etching is the use of a retarding agent (see separate entry Retarded surface finish) during casting or grit blasting (see separate entry Grit blast finish) the hardened concrete.
WARNING: Acid etching must be carried out strictly in accordance with the appropriate COSHH Regulations and in accordance with the manufacturer’s recommendations.