Using gel acids to stencil patterns on concrete

Concrete Construction, Dec, 2002 by Joe Nasvik

Stencil sandblasting is one way to create delicate, intricate patterns on the surface of concrete. Another way involves placing hydrochloric acid suspended in a heavy gel over stencil patterns adhered to the concrete surface. Introduced as a proprietary product by Surface Gel Tek, Scottsdale, Ariz., under the trade name Tek Gel, two advantages of this system are that it is less messy than sandblasting and the profile of the relief is very uniform and not as deep. According to Tamryn Doolan, the company's general manager, the gel acid removes the "cream" (cement paste) of the finish on a concrete surface, exposing fine sand and imparting a soft, suede appearance.

How the process works

Just about any concrete surface, new or old, is a candidate for the process. The most important requirement is that there be no sealers or coatings that would prevent the acid from penetrating into the surface. The finish of the concrete can be either hard-troweled or fairly rough, but it should be clean and free of standing water.

Vinyl stencils with an adhesive backing are generally recommended over other materials. Several companies now manufacture them, including Surface Gel Tek. Stencils can be "off-the-shelf" or custom-made for a single application.

Place stencils on the concrete floors or walls in the same way you do for sandblasted finishes. Be sure that there is good adhesion to the concrete. If there is any danger of accidentally getting gel acid on surrounding areas, carefully mask them.

The next step is to apply the gel acid with paintbrushes, squeegees, sprayers, or rollers. The acid is mixed into a heavy gel substance so that it doesn't move around or "bleed" under the stencil after it has been applied. Nevertheless you still have to exercise some care not to work it under the edges of the stencil during application. For troweled floors, about 350 to 400 square feet per gallon is the expected coverage. Coverage for rough surface floors is about 250 square feet per gallon. Normally only about 10 minutes are required to complete the etching process. The resulting finish will be equivalent to 120-grit sandpaper.

Doolan recommends leaving the stencil in place until the spent gel acid has been removed. If there is a floor drain, the material is easily hosed off. On flat, level floors the excess material can be scraped into a pile and then placed in a plastic bucket. She advises putting some baking soda in the bucket to neutralize the residue to make it safer to handle. Spraying the slab with clear water and wiping it up with a clean cloth or sponge completes the process. When the slab is dry, it can be sealed with a suitable sealer.

Coloring

As in the case of sandblast stenciling, chemical stains can be applied before or after stencils are placed. Stains placed beforehand will not be entirely removed by the acid, although the color intensity will be changed and a texture will be created. If stains are placed afterward, stencils should be left in place until after the staining process is complete.

Benefits of the system

Stencil patterning with gel acids provides a more gentle, subdued effect than sandblasted images. There is no noise or mess that would limit other trades from working in the same area. There isn't any air pollution either, though there will be a little smell from the acid.

For information on Surface Gel Tek, call 480-970-4580, e-mail surfacegeltek@qwest.net, or circle 6 on the reader service card.