Concrete Staining Step-By-Step
Masking Walls and Woodwork
Before cleaning the slab, carefully mask off every surface contiguous to the stain area with plastic sheeting and a high-quality painter’s tape. Acid is a corrosive and invasive compound. It will affect almost any surface it touches, and walls and woodwork are especially susceptible. Acid, and the metallic salts which make it stain well, will turn wood black, corrode metal, and even stain porcelain. (If your walls are accidentally stained, cover these spots with a lacquer-based, stain-blocking primer like Kilz before repainting.)
Plaster walls require a bit more prep than drywall. These walls are best protected when they have been finished with their topcoat and sealed with an invisible waterproof sealer. If you have any doubt about the resistance of any surface to the acid stain, then mask it.
Cleaning the Slab
I can’t stress enough how important it is to thoroughly clean the slab before staining. The condition of the slab is so vital that I estimate wall masking and floor preparation to be 80 percent of the labor of staining. Any spot of paint, tar or mastic left on the slab will mar your stain finish.
To clean the slab after masking, first scrape away any drywall mud from the slab and vacuum up debris. Use TSP (TriSodium Phosphate, sold in the paint department at hardware stores). Use about 2 Tablespoons of liquid TSP or TSP Substitute to 4 gallons of water to power-scrub the entire floor, using a power rotary floor buffer fitted with a coarse black pad. Rinse well. Do corners and edges by hand. If plaster residue or latex paint is spotting the slab, it will be necessary to soak these sections for 40 minutes, then power-scrub. Follow the power scrubber with a wet-dry vacuum, and mop behind the vacuum with clean rinse water. A newer type of mop made with tubes of microfiber fabric sold at janitorial supply stores will work twice as well as the old type of string mop. Work small areas at a time, otherwise concrete slurry is likely to dry before you get to it. Dried slurry will show through the stain.
If your first TSP scrub reveals spots of white paint or black tar on the floor, scrape them up with a 3-inch wide razor scraper and clean off the residue with a bit of lacquer thinner or xylene rubbed on with a rag. Wear chemical gloves and a mask for this part; most smelly solvents are carcinogenic. (Even if there is enamel or oil-based paint on the slab, do not try to use paint thinner. It doesn’t work well and leaves an oily residue on the slab.) You want nothing oily or waxy on your clean slab, since the acid stain will roll away from those spots. TSP is great for cutting through oily spots.
Filling Holes and Cracks
Once you have properly cleared the debris and power-scrubbed the slab, you can move on to filling holes and cracks. It’s fairly easy to judge what needs to be filled—anything wide enough to hold a credit card. Hairline cracks are of no concern; they add to the overall interest of the surface.
First, remove any “islands” and loose particles from the crack, using a cold chisel. You want to create a void within the crack to allow proper filling. Vacuum the crack. Inject concrete glue into the crack, wiping up any excess. Wait about 30 minutes for the glue to set up.
Mix a cementitious floor topping compound, which comes in dry form from your concrete supply store, to a “heavy cream” consistency by adding water, and then press it into the crack with a spatula. When the filler has dried, scrape off the high spots, and sand along the crack with a drywaller’s sanding screen.
Note that our staining company does not use products that are sold as patching compounds, because they are generally too sandy and rough. Most patching products sold at home-improvement stores will create a texture that is obviously at variance with the rest of the slab. Also, do not use latex or acrylic to fill cracks; it will not accept stain.
If filled areas are extensive across the floor, it may be necessary to scrub and rinse the entire slab again, with the rotary floor buffer and a red (medium) pad. If filled areas are infrequent, residue can be scrubbed from the sides of the cracks by hand with a piece of plastic steel wool and some water, sponging up the filler when it is still in suspension. Filler should only be seen in the cracks and holes and not as a wide dusty swath surrounding the repair, as it will take the stain color differently. Most fillers accept stain but look much lighter than the rest of the floor. They often need to be touched up later on with artist’s acrylic paints mixed to match the surrounding floor.
Finally, the Staining
The method of application depends on the look you want. If you want an evenly stained, “leather” look, you will need to apply stain with a sprayer. We use a pump-up garden sprayer. Filter the stain through a paint strainer as it goes into the tank. Otherwise, small particles can clog the spray tip and lead to a very spotty stain job. The sprayer should be set to a fine mist. Be sure to buy a sprayer that has all-plastic parts. Any metal parts in a sprayer will quickly corrode from the acid, and a metal tank may even explode!
When the floor is dry enough to walk across, you will need to spray again, and perhaps a third time. Eventually, the spray speckles will integrate into a uniform looking floor color. If you are spraying a very reactive slab, you might first have to dampen the floor with a well wrung mop. The spray speckles will spread and even out on a damp floor.
Acid stain can also be applied by pouring. Staining this way is definitely a two-person job. If you pour on the stain, be sure that one worker prepares a wet spot of water on the slab into which the second worker pours the stain. Otherwise you will have a darker spot where the fresh stain sat and reacted before you could spread it out. The pouring technique uses more stain than spraying, but gives a marbled look that we find quite interesting.
If texturing devices are being laid into the wet stain, a third worker should be on hand to accomplish that. Whether you are laying down “veins and rags,” thin plastic, or alfalfa or straw soaked in a darker color; nothing will make a mark if the floor is already dry.
Before loading all of your tools and buckets onto the slab, get a small cart—a child’s wagon will do. Use this to shuttle your buckets and tools around the slab; the bottoms of buckets and stain bottles can leave rings on the prepared floor. We use one rolling cart for our stain implements and another cart that holds a five-gallon bucket half full of water. This is where we place the staining brush during pauses. Each person in your crew will need a respirator fitted with a cartridge rated for inorganic gases, specifically for chlorine-type gas, which is a byproduct of the staining process. They will also need skin and eye protection.