Preparing Concrete

Your job here is simple to say and hard to do: get the contaminants out of the bond line. Not profile it, not grind it. Clean it.

Most of what follows feels backwards, because good advice usually does. Burn in three things before you start:

  • Dry-grinding and pressure-washing drive surface oil deeper into the pores. They hide the worst contaminants instead of removing them.

  • Chemistry makes it worse, not better, unless you get the solution off the floor. A cleaner lifts the contaminant into suspension; let it dry and it drops right back. The floor is as dirty as ever until you squeegee or vac the solution up.

  • Water is your X-ray. Wet the slab — wherever it stays light or beads, something is in there to remove.

A few contaminants show up on nearly every slab, so you start there. The rest are almost infinite, but they fall into a handful of families, and I will give you the play for each. Twenty years of paying for other men’s mistakes taught me one thing: complex instructions collapse into sloppy work, and simple instructions free a man to think. Learn this, put it in your own words, and apply it with excellence.

Removing the salts that are on nearly every slab

Nearly every slab of concrete sat outside for months while they built the building, got wet, got dusty, and dried out over and over again, so you'll have to remove the minerals that were in that drying water for sure. It’s like if you parked your vehicle outside for 3 months, it would have a white haze on the paint. Concrete also exudes calcium carbonate (CaCO₃), sodium sulfate (Na₂SO₄) and potassium sulfate (K₂SO₄) when it cures.To get that off, any acidic cleaner will do, but the more acidic it is, the more you will need to rinse it to remove the residue from the reaction between that acid and the concrete. Traditionally, acid has been used to create a concrete surface profile. That’s not what we are doing here. We are just using the acid to get stuff off. We made CleanPrime Detergent for this purpose, but other methods may work.

As you work in the solution of CleanPrime Detergent and water and then rinse it with more water, you will see other contaminants better than you could any other way. Clean concrete drinks water in and darkens evenly. That even, dark wet-out is your baseline. Anywhere it stays light or beads, mark it. That is, stains that repel water may also repel the primer. Below is a video of me beginning to wet-scrubbing the office of the plant in Texas where we make this stuff.

One important thing not shown in the video above is using the squeegee well. Picking up the solution on the floor is like cleaning a windshield at a gas station: the water can be super dirty, but as long as you pull that squeegee mindfully, you can get it really clean.

Removing the salts as described and shown above needs to be done on almost every slab. Below are things you might see and how to handle them.

  • Why it wrecks a bond. Loose, weak material on the surface. Your coating bonds to the crud, the crud knocks loose, and your coating goes with it.

    How to get it out. A buffer with a black pad does wonders with this stuff. Tilting the handle up on the left and down on the right will push it back into your right hip flexor and make the right scrape and grind it flush, then wet-scrub the dust and chalk off. Drywall mud and gypsum come up fast; thinset and slurry want a scraper or grinder. Then it joins the every-slab wash.

  • Silicone is the meanest contaminant you'll meet, and it won't wash off with detergent. You need a solvent-based wax, grease & silicone remover — the kind a body shop wipes a car down with before paint. Get the gallon.

    This stuff is flammable. Everything from the safety page applies here, doubled: kill the ignition sources, move the air, glove up. Again, a properly fit-tested respirator with organic-vapor cartridges carries you. In a tight basement, an occupied house, or anywhere the air just won't move, step up to a PAPR

    The trick is two rags. One to flood, one to lift. Here's the order:

    1. Find the spot. Wet the floor first. Where the water crawls and beads, that's your silicone. That's where you work.

    2. Flood it. Soak the spot with the remover — be generous. You want it wet enough that the silicone floats up off the concrete and into the solvent.

    3. Agitate it. Work it in with a brush, especially on a porous or rough floor where it's hiding down in the pores.

    4. Lift it — fast. Wipe it up with a clean rag, then immediately go behind it with a second, dry rag before the solvent flashes off. If you let it dry, the silicone just settles right back down where it was. One rag smears the problem around. Two rags carry it off the floor.

    5. Fresh rags, every pass. A dirty rag is just a silicone paintbrush. Flip to clean cloth constantly.

    6. Re-wet and check. Pour water on the spot again. If it lays flat now, you won it. If it still crawls, hit it again — bad spots take three, four, five passes, and that's normal.

    7. Let it rest, then re-check. On a floor that's seen years of tire shine, walk away for a day or two and test it once more. Silicone bleeds back up out of the pores when you're not looking. Better it shows up now than under your coating.

    8. When it won't quit: if a patch still beads after you've cleaned it good and let it rest, the silicone has soaked too deep to wipe out. Grind that spot down past it, clean it once more, and re-test. Don't coat over a spot that won't take water flat. It will not heal. It'll just become the place your floor peels.

  • DO NOT TRY USING DRY GRINDING OR HEAVY ACID ON OIL - IT MAKES IT WORSE. 

    Protocols listed according to how deeply the oil has penetrated the concrete. Consider this like three rungs on a ladder down into the concrete. No matter what,  you want to scrub in cleaner, let it dwell, rinse with clean water — and use fresh solution every pass). 

    1. Fresh, shallow grease gives up to high pH. Alkaline cleaners turn oil into soap so it rinses away instead of clinging.  SureBest 2790 Con-D-Soil is the best alkaline cleaner I know of. Kemiko NeutraClean uncut is very good as well. 

    2. When the alkaline wash isn't quite enough - it’s still repelling water - step up to a citrus (d-limonene) or strong butyl degreaser and agitate it like you mean it. But here's the trap nobody warns you about: most degreasers don't actually remove oil — they thin it and spread it so it looks gone

    3. Use a poultice extraction for deep contamination. A poultice is just two things working together — something wet that dissolves the oil, and something dry and hungry that pulls it back out of the slab. You're using the concrete like a sponge in reverse.

    This cannot be repeated too much: extraction is the point. A degreaser doesn't kill oil. It lifts it. It breaks the oil loose from the concrete and floats it up into the water, held in suspension. Think of it like a taxi — it picks the oil up off the slab, but it'll drop it right back where it found it the second that water dries. So if you scrub, walk off, and let the floor dry, you didn't clean a thing. You just gave the oil a ride around the block and dropped it home. The whole game is hauling the taxi off the floor while the oil's still riding in it.

    So, 

    • Scrub and agitate the degreaser in — hot water seems more effective, but it’s inconsistent because the heat will quickly be lost to the concrete. Keep it cool and consistent.

    • Don't let it dry. Pull it off wet. This is the most important thing. While the slurry's still suspended:

      • Big floor: an auto-scrubber earns its rent — it scrubs and vacuums the dirty solution up in one pass, so the oil never gets the chance to settle back down. A walk-behind from Tennant, Nilfisk, Clarke, any of the real ones.

      • Small spot, or no scrubber on the truck: squeegee to a wet vac. You can throw an absorbent straight into the wet slurry — oil-dry, diatomaceous earth, even cat litter — let it drink up the oily water, then sweep it and bag it. I’ve personally never done that: if I was too cheap to rent or have an autoscrubber, I’m certainly feeling too cheap to use up absorbent. 

    • Rinse with clean water and extract again. Fresh water every single time. The first pull never gets it all; it's the rinse-and-extract cycle that chases out the residue. And never push dirty water around with a mop - that is just repainting the oil back on. 

    • Repeat until the water you pull up comes back clear and the spot passes the bead test — water laying flat and soaking in even.

    • Let it dry, then check it again the next day. Oil bleeds back up out of deep pores. Better it shows up on your water test than under your finish.

    Short version: the degreaser arrests the oil, but it's the wet-vac or the scrubber that hauls it to jail. Skip the haul, and you've done nothing but stir the pot.

  • What it is. Somebody put a film between you and the concrete. On a new slab it's usually a curing compound — sprayed on fresh to hold water in while it cures, which it does by sealing the pores shut. On an older floor it's a sealer (a topical acrylic or urethane skin, or a penetrating one) or form-release left over from the pour. Different products, same problem: a barrier.

    How to spot it. Water should darken the concrete when it soaks in. With a curing compound, water beads, it stays light, and the CleanPrime Detergent isn’t fizzing at all.

    Why to remove it? These films are built to be easy to strip. Your epoxy is built to be impossible to strip. Bond to the film instead of the concrete and your strong coating is only as permanent as the weak layer under it

    How to remove it. Mechanically, it's probably the best way, though you can remove it chemically. Below are some links

  • Decline this project. This stuff is incredibly bad for coatings. Over time the colloidal silicate or silica will continue to grow crystals and push any coating off. There is no known way to stop this if moisture vapor is coming up through the concrete (very common).

  • STOP — TEST FOR ASBESTOS FIRST. If the building predates the late 1980s and you have black, tar-like adhesive or old tile mastic, treat it as asbestos until a lab says otherwise. You cannot tell by looking. Disturbing it dry can put fibers in the air, and if it tests positive, removal is a licensed abatement contractor’s job by law — not yours.

    What it is. Old flooring glue left after carpet, VCT, or tile came up. Two kinds of mastics to deal with (not asbestos - again, if it’s that, do not mess with it). 

    Brittle adhesives (most often light-colored).

    For these, carbide scrapers on a planetary floor grinder are hard to beat. They don’t tear up the concrete and make a mess of dust or slurry, but they scrape off with a lot of force multiplication. PCDs are a close second - you must run wet with these and adding a low-foaming degreaser

    Gooey adhesives (aka cutback; asphalt-based usually black and tarry).

    These are tricky - you need something to break it down, and then the more important part is to clean off the broken down residue. The most impressive company in this space in my opinion is Franmar, and while I can’t tell you which of their strippers is ideal for the mastic on your project, I can tell you to go very hard with their 700DG: EMERGE™ DEGREASER is hard to overuse after using any of the chemicals that they make/sell. 

  • What it is. Living growth — mold, mildew, algae — feeding on moisture and whatever organic film is on the slab. It loves shade, damp, and dead air: basements, north walls, under old flooring.

    How to spot it. Black, green, sometimes white or orange blotches; often slick or fuzzy, usually musty. Do not confuse white mold with efflorescence — the salt dissolves and wipes away with water; the mold does not.

    Why it wrecks a bond. It is a living layer with no structural grip, and it is a moisture flag. Coat over live mold and you have sealed a food-and-water source under your film; it keeps growing and eating at the bond line while the moisture that fed it is still driving up through the slab. (That is the exact moisture story CleanPrime is built for — but the growth still has to go.)

    How to get it out. Kill it, lift it, then deal with the water. Wet it with a mold-killing solution — household bleach cut about 1 part to 10 (or oxygen bleach / sodium percarbonate, or 3% hydrogen peroxide if you would rather skip the fumes) — dwell 15 to 30 minutes, scrub, then rinse and extract the dead growth off the floor. Rinse well: bleach leaves a salt behind (back to your salts rule). Never mix bleach and vinegar — that is chlorine gas. Then dry it.

  • What it is. Iron soaked into the pores — from something metal left sitting on the slab, from rebar or wire too close to the surface, or from iron in the original mix water.

    How to spot it. Orange to red-brown staining. Detergent and pressure washing will not budge it, because it is not on the surface — it is a chemical bond down in the pores.

    Why it wrecks a bond. A light, stable rust stain in sound concrete is mostly cosmetic. But loose rust scale will not hold anything, and rust that keeps returning is telling you steel is corroding below and the slab is staying wet — both are bond-killers, and the second is structural.

    How to get it out. Oxalic acid. Gentler than it sounds (it is what is in rhubarb) and it works by chemistry, not muscle — the oxalate grabs the iron and pulls it up out of the pores into solution. Mix about a cup per gallon of warm water, flood the stain, dwell, scrub, and rinse it all off BEFORE it dries — same gospel, get the solution off the floor or the rust resettles. Rinse until no white residue is left. Stay away from anything with hydrofluoric acid or "ammonium bifluoride" — that is a far more dangerous league.

  • There are 2 ways to remove paint from concrete.

    Chemically: like mastic, you use a stripper to break it down, then a detergent to get the broke down components off. If using Franmar materials, their 700DG: EMERGE™ DEGREASER is hard to overuse after using any of the chemicals that they make/sell. 

    Mechanically: Most folks grinding off paint might be better off with a combination of carbide scrapers and a stripper or PCD tooling.

  • This is less heavy-duty than a coating, so if you detect this, you're in for less work. You man notice a built-up sheen worn thin in the walking lanes and thick at the edges; it scuffs and yellows. It is a film, so water beads.

    Strip it like a janitor would, just thoroughly. Flood on a floor stripper, keep it wet (re-wet as you go — do not let it dry), dwell, then scrub with a black pad and pull the slurry up with an auto-scrubber or wet-vac. Multiple passes until it is all gone, then a clean-water rinse to get the stripper residue off.

    Again, you are in charge of your job, but Ecolab "Fast Action Floor Stripper” from Home Depot worked astonishing well for me - here’s a link

The bottom line is get everything in solution, then get the solution off the floor.

An old carpenter once told me “Cory, you’ll never know if you built it too strong.” So, when in doubt, clean more.

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