Surface preparation is the most important and time-consuming part of any stained concrete flooring project. To look great and hold up over time, the flooring project must begin with a surface that is clean and sound. The best way to get concrete clean is diamond grinding. The key to ensuring its soundness is making all the appropriate repairs.
Getting a concrete floor clean may mean using a variety of methods, ranging in aggressiveness, depending on the age and condition of the floor. Almost all concrete slabs, unless they are brand new and have been protected, require a great deal of cleaning. The concrete may previously have been treated and require the removal of carpet glue, tile adhesive, paint or gypsum underlayment. Even if the concrete has not been treated, there is still always a great deal of drywall mud, paint, primer, insulation overspray, solvent spills, glue and other construction adhesives to be found on the surface, because builders and tradesmen are on a tight time schedule and rarely take time to protect "ordinary", gray concrete.
Concrete diamond grinding, done properly, shaves off whatever is on the surface of the concrete while affecting the concrete itself, as little as possible. Materials like carpet glue, tile adhesive and paint generally need to be ground off. Chemical strippers can be effective in removing tacky or sticky adhesives that may clog grinders, and may be helpful at the outset of a project, but they will not get all residue off the floor and need to be followed by light grinding to get a concrete floor clean enough to stain. Also, while strippers may be effective in removing thin layers of paint and sealer, and small, isolated areas of adhesive, the oils and solvents in these strippers can permanently stain or darken the concrete, as can the carpet glue or tile adhesive itself. Grinding is the best way to remove all traces of a substance from a cement surface without compromising its integrity.
More aggressive methods to clean concrete, such as shot-blasting and scarifying, are required for removing thicker layers of material, like epoxies, gypsum-based underlayments, thinset mortar under ceramic tile, and raised concrete "caps" installed to cover up electrical or plumbing repairs. Shot-blasting or scarifying may also be necessary to clean concrete with a very rough texture or embedded staining. These methods do compromise the cement surface of a concrete floor, and need to be followed by grinding to smooth the surface. These methods may require a concrete overlay, also, to cover scratches, furrows and exposed aggregate.
Getting concrete clean does not always require grinding. If concrete has not previously been treated, scrubbing with a rotary floor machine, using a black stripping pad, cleaning detergent, and warm water is also effective. A couple passes using this process will get up the majority of dirt, dust and drywall mud. Anything left behind will then show up clearly enough on the wet concrete, and be sufficiently softened, to be removed with a 4" wallpaper scraper. An angle grinder fitted with a lightly abrasive sanding disc can also be used for spot removal. Unfortunately, some blemishes caused by chemicals and solvents during the building process cannot be detected during cleaning, and only show up after the staining process. These blemishes can only be fixed by engraving them out, and touching up with stain, or through "faux finishing".
Making concrete sound enough for the installation of acid-stained and other decorative concrete flooring, like polished concrete, metallic epoxy and overlays, means using one or more of three applications: 1) patching small holes and divots; 2) crack repair; and 3) rebuilding of damaged or missing sections of concrete.
Patching is necessary on almost every concrete floor prior to staining. Small chips or missing pieces of concrete are commonplace, and may look natural enough not to need to be addressed. But larger spalling, and holes left by nails from carpet tack strips or the removal of bolts, does stand out. This type of patching can effectively be done with cement grouts and epoxy repair materials. But these materials require at least several hours to dry, and in some cases need to sit overnight. This may not present a problem if there is a great deal of patching to be done. But if it's only a small amount, and the next step of a flooring project needs to proceed quickly, there are specialized, structural, urethane and polyurea repair products than can be used, which dry in a matter of minutes.
Cement grouts and epoxies can also be used to repair cracks, but since crack repair is a two-step process, urethane or polyurea repair materials are preferred since they set up quickly, and the second step of the process - removing excess material from the surface - can begin almost immediately. There are "structural" urethanes and polyureas for stable cracks, and "semi-rigid" ones for cracks that may continue moving. The excess material from structural repairs must be ground off, while excess material from semi-rigid repairs has to be shaved flush with a long-handled razor scraper.
The rebuilding of damaged or missing sections of concrete is most often required near joints and around the periphery of a concrete slab. It may be done with either new cement or repairs mortar based on cement, epoxy or polyurea. Cement and cement mortars are inexpensive, but they take a long time to cure. Epoxy and polyurea repair mortars are more expensive, but they dry much faster.
Since patches, crack repairs and rebuilt sections of concrete usually do not blend in color with the main body of a concrete floor, it is often important to mix color with the repair material beforehand, if the concrete floor is going to be stained. That way, remaining imperfections may not stand out, or faux finishing techniques may be used to conceal them. Even if the repairs do stand out, for example after plumbing and electrical repairs or upgrades, the overall appearance may not be objectionable, if the repairs are few in number, or they be incorporated into the overall design of the floor. As with drywall mud, paint and other construction contaminants on a concrete floor, much of the damage happens during the build-out process and may be prevented by using a temporary, protective floor cover covering.