Even for a contractor with years of experience, a concrete staining job can be affected by outside factors. When it comes to staining, the most common problems are lack of penetration, color variation and light coloring. Several factors can contribute to these staining issues, and most can be prevented with a little extra prep work and open lines of communication with the customer and other contractors on the jobsite.
If you know there are concrete or environmental issues that may affect the stain's final color, discuss them with the customer. Let them know what possible issues could occur, why they could happen and their options for preventing these issues.
Sometimes the stain manufacturer can assist with communication efforts, says Scott Thome, director of product services for L.M. Scofield Company. "The manufacturer can talk to the owner about chemistry and how a product will work on a particular job," he says.
If you're working with a general contractor, or if other subcontractors will be working on or around the floor before it is stained, communication with these contractors is essential. Let them know that tire tracks can show through a finished stained floor. Marking the concrete slab with magic markers or pencils may prevent a stain from penetrating in these areas. Materials such as paint and oil can also prevent penetration or show up after a stain is applied. Subcontractors should also know that if anything is left on the slab during curing, it could create a shadow which the stain won't hide - and may even magnify.
Know the concrete
Before applying stain, survey the jobsite and locate any issues that may exist. The type of concrete affects how a stain takes to it. "Was it a 5,000 psi? Was it a high polished concrete? The tighter the surface of the concrete is, the harder it is for any stains to penetrate and create color," says Buster O'Steen, product specialist and trainer for McKinnon Materials. Certain types of stain may be better suited to tighter concrete. Some water-based colors will have larger pigment grains than others, and in tighter concrete those colors won't penetrate as well, he adds.
The concrete mix also needs to be considered. Fly ash in a mix, for example, can alter the concrete and affect the chemical reaction of acid stain, Thome says.
The condition of the concrete plays a big role. Is the concrete new or old? If it has been in use, what was it used for? Is it a commercial or residential jobsite? Has heavy or light traffic traveled on it? What types of contaminates could have entered the concrete?
Proper cleaning before applying stain can help eliminate problems. Thome suggests using a silicone carbide impregnated nylon brush and water to clean the concrete and then vacuuming it all up. A little non-film forming detergent can also be used to lift grease from the floor, he adds.
If the concrete has already been sealed, or a cure and seal has been applied, it will have to be removed with a grinder before staining. Stain will not penetrate the sealer, so staining over a sealer will diminish the color or result in no color at all. Some strippers might alter the pH of the surface. A mock-up is always a good option when trying something new, Thome says.
Concrete should also be tested for moisture content prior to applying stain. In some cases, too much moisture can result in poor penetration, diminished color or no color. O'Steen suggests using an old rule of thumb for testing moisture. "If I wet my thumb, put it on the concrete, and the concrete changes colors, I can penetrate it because it's drawing moisture in," he says.
Check the pH levels of the concrete. "Your pH should be north of 10.5 on the pH scale," Thome says. "In low pH areas some of the darker acid stain colors may not take as well because the pH level is too low to neutralize the acid, and the darker colors tend to have more acidity than lighter colors," he adds.