A popular technique is to start with a light concrete base color (whether an integral color or a color hardener) and then apply a much darker release agent for contrast. Although about 70 to 80 percent of the powdered release is washed away after the concrete hardens, the remaining release becomes depressed into the surface paste during stamping, which creates the subtle color accents.
Powdered release agents are more traditionally used by stamping contractors because they offer more color selections. However, they do have disadvantages. Because these very fine powders are dusted onto the concrete surface, they create airborne dust particulates, so workers must wear dust masks to prevent inhalation. On windy days, the airborne powder can stain nearby buildings, existing concrete flatwork, and landscaping, making it necessary to mask off adjacent areas with plastic or paper sheeting. Because of these concerns with powdered releases, more contractors are using clear liquid releases and tinting them with a powdered release (see the tip below). This method is quite effective and eliminates the disadvantages of using a powder alone.
I’m from the old school, though, and believe you can obtain a much more realistic look using a straight powder release. It’s often a trade--off: Powders require more cleanup and masking of adjacent surfaces, but they produce greater contrast than a liquid release.
Tip: Generally, one cup of powdered release is needed to tint approximately 5 gal. of liquid release. A helpful hint: Add the powder in increments, maybe 1/2 cup at a time, until the tint level you want is achieved. Mix the powder with the liquid release, and then dip a clean rag, sponge, or brush into the tinted liquid and drag it across a piece of white paper to check the color intensity of the tint. If you decide you need to intensify the tone, add another small dose of powder. It’s much easier (and less wasteful) to build up the tint level this way than to add more liquid release to soften a tone that’s too bold.
Some stamping contractors like to achieve color variegation by applying chemical stains to the concrete after it has cured. Acid--based stains react chemically with the concrete and produce a mottling effect that gives your stamping work a sense of realism. If you’re trying to mimic the color variations of natural stone, an acid stain will allow you to achieve interesting highs and lows. Most stains can be diluted to achieve varying degrees of color transparency. You can also apply stain randomly to individual stones in the stamped design — something that would be impossible to do with color hardeners or releases that are worked into the concrete surface.
Another reason to use a stain is to mask blemishes. No matter how careful you are, it is difficult to produce a blemish--free stamp job. There will always be some subtle color imperfections. For example, maybe the pigmented release didn’t take completely in one area. You can use a stain to accent the color in that spot and disguise the flaw.
Dyes and tints
Dyes can be used in conjunction with stains to achieve greater color intensity. They can produce bright, vibrant colors, and you can mix your own custom colors on the jobsite to broaden your palette.
Unlike stains, dyes are not chemically reactive with concrete. Instead, the fine coloring agents in dyes penetrate the concrete surface. Keep in mind, however, that dyes will fade somewhat (generally 5 to 10 percent) when exposed to ultraviolet light. They are not as UV--stable as acid stains.
Tints, which are generally diluted color washes, can be used to add hints of color and produce some interesting faux finishing effects. There are several methods of creating tints. Probably the most popular is to mix a pigmented powdered release with a solvent--based acrylic sealer, so your sealer itself acts as a color wash. A word of caution: This method won’t work with water--based products; you need the solvent to break down the particles in the powdered release.
I’ve also seen contractors make tints by mixing several handfuls of color hardener with water in a pail. The heavier solids in the hardener settle to the bottom of the pail while the pigments color the water. The tinted water can then be applied to the concrete by spray or sponge. Keep in mind, however, this type of tint is not as permanent as a stain or color hardener. You will definitely need to lock in the color by applying several coats of sealer.