Floor Preparation

Preparation is most of the job. It is the physical part, the dusty part and the part with nothing to show for it, and it is the part that decides whether the floor holds. A perfectly mixed, perfectly applied coating over an unprepared slab fails. A slightly uneven coating over a properly prepared slab does not.

Adhesion is mechanical

A floor coating does not glue itself to concrete chemically. It flows into the open pores and surface texture and cures there, and the strength of the bond is the strength of that mechanical key.

A troweled slab has almost none of it. The surface is closed, it is often coated in a curing compound, and it is topped with laitance — a weak layer of fines and cement paste that rose to the top when the concrete was placed and dried there. Coat that surface and the primer bonds beautifully to the laitance, and then the laitance lets go of the concrete. The floor comes up in sheets with a thin gray skin of your slab attached.

Preparation removes the weak layer and opens the pores. That is the whole objective, and everything below is in service of it.

The short version

You are going to rent a floor grinder, cut the glazed skin off your slab, fix what that exposes, and vacuum until the floor is surgically clean. That is the whole job. It takes most of a day on a two-car garage and it is the single thing that decides whether the floor is still there in ten years.

Why grind at all? Concrete cures with a smooth, dense skin on top — laitance. Coating sticks to it perfectly well, and then the skin lets go of the concrete underneath, taking your floor with it. Grinding removes that skin and leaves an open texture the coating can key into, roughly like 60–80 grit sandpaper. That texture has a standard, and ours is ICRI CSP 3–5.

You will not be acid etching. A rental grinder is the method this guide teaches, and the only one we recommend for a garage you are doing yourself. The comparison below explains why, because it is worth understanding rather than just being told.

What the data sheet says

The resultant surface should be free of laitance and efflorescence and have a surface texture similar to medium (60-80 grit sandpaper).

Manufacturer product data sheet, surface preparation

Read the order of that sentence. Three mechanical methods, and then acid etching, qualified twice — “in some instances”, and “check with the manufacturer”. That is not a menu of equals, and we do not treat it as one.

In practice the choice is already made for you. Somebody coating their own garage rents a grinder; the other methods need equipment or licensing that does not come out of a hire shop, and if a slab genuinely needs them it needs a contractor rather than a weekend. So this guide teaches grinding, and the table below is here to explain what you are not missing.

Why not acid etch?

The last two rows are the ones people underestimate. Etching finishes with a wet floor, a residue that has to be neutralized and rinsed until it is gone, and water driven into a slab you are about to seal — which is the problem described in concrete moisture, created deliberately on purpose the week before you coat.

What ANVIL recommends

Grind. A diamond grinder with a vacuum attached, worked over the whole floor and then into the edges and corners by hand, is the preparation this system is designed around. Shot blasting is equally good and much faster on a large floor; it is usually a rental or a contractor, and it still leaves the edges to a hand grinder.

Budget four to eight hours for a two-car garage, and be ready for more. How long grinding takes is decided by the slab, not by the square footage: a soft, clean, recently poured floor comes up quickly, and a hard old slab with a curing compound on it can take a full day on its own. This is the step people underestimate and then rush, and it is the step that decides whether the coating stays down. Give it its own day rather than trying to grind and coat in one.

We are not going to tell you that you cannot acid etch. We are telling you that you should not. The data sheet allows it in some instances, and our answer is still no — the same way a doctor can note that something is technically your decision while being perfectly clear about what they advise.

Acid does not profile concrete. It dissolves the surface and leaves the residue behind, so what you get is a slab that feels rough and is chemically contaminated — which has to be neutralised and rinsed, and then has to dry completely, and moisture is the one thing that will take the coating off from underneath. It cannot reach the profile this system is built for, and it cannot touch a curing compound or an old sealer at all. Every failure it causes appears weeks later, after the floor is finished and the mess is expensive.

If you believe your floor is a genuine exception — the space cannot take dust, or a grinder physically cannot reach it — talk to us before you buy anything. There is usually another way, and it is a five minute conversation.

Grinders and shot blasters are day rentals almost everywhere. Take the dust seriously: concrete dust is respirable silica. The coating carries a California Proposition 65 warning of its own, though for titanium dioxide rather than quartz. Use the extraction, wear eye and hearing protection and a proper respirator, and do not dry-sweep or leaf-blow the dust when you are done.

What a prepared floor looks like

• Uniformly dull. No shine anywhere. A shiny patch is unprepared concrete, a sealer the grinder skated over, or a low spot the machine never touched.
• Rough to a bare hand, something like medium-grit sandpaper.
• Profiled at the edges, not just in the middle. Edges and corners are done by hand, they are the easiest place to hurry, and they are where floors start to fail. They will not match the field and they are not supposed to: a walk-behind grinder gets the open floor to about CSP 4, while a hand grinder in a corner gets you about CSP 3. That is the honest target. Chasing an identical texture at the wall means grinding until you have taken off more than you needed to.
• Absorbent. Drop water on it. It should darken and soak in within seconds, everywhere you try. If it beads or sits, something is still on the slab.

The trade describes this on a numbered profile scale, and the sheets do now specify one: ICRI CSP 3–5. The sheets put the same thing in plainer words — the prepared surface should have a texture like 60–80 grit sandpaper, cleaned to ASTM D 4258 and profiled to ASTM D 4259. The four checks above remain the practical test, and the water drop is still the one that settles arguments.

Oil goes first

Degrease before you grind, not after. A grinder run over an oil stain drives the oil deeper and carries it across the floor on the diamonds, turning one contaminated patch into a contaminated floor.

1. Scrape off anything sitting on the surface.
2. Apply a concrete degreaser, scrub it in with a stiff brush, leave it the time the label says, and rinse. Repeat anywhere still dark.
3. Let it dry, then test with water. Water that beads or sits means oil is still there.
4. Grind, then test with water again. Oil below the surface only shows itself once you cut into the slab.

The primer tolerates the surface oil and grease a real garage slab has in it, but nothing seals oil in. Contamination that has soaked deep into a slab — the spot under a vehicle that leaked for years, the ground beneath an old compressor — can keep wicking up after the floor is coated and show later as a soft or unbonded patch. If a stain keeps coming back after degreasing and grinding, stop and talk to us before you coat over it. Occasionally the honest answer is removing that piece of concrete.

Cracks, spalls and patching

Repairs happen after grinding and before coating. Patching material bonds far better to opened concrete than to a sealed surface, and grinding a repair that is already down mostly removes it.

• Static cracks get chased out into a V with a crack chaser or a grinder blade, so the filler has something to hold onto. Vacuum the dust out of the crack, then fill flush with the floor.
• Spalls and pitting get filled and levelled. A pit you leave is a pit in the finished floor. Flake does a lot of things well and hiding a hole is not one of them.
• Every repair has to be cured before you coat over it, and compatible with the coating going on top. Read the label, or ask us.

The repair materials ANVIL will supply are not published yet — ANVIL patch and crack-repair materials not yet published — so if your floor needs more than a light patch, ask before you buy anything to fix it with.

One crack is not a patching job. A crack that is moving, that has a height difference between its two sides, or that runs the full width of the slab, is structural. Filling it does not fix it and it will come back through the coating. Send us a photograph before you order.

Control joints

The straight tooled or sawn lines across your slab are control joints. They are deliberate: the concrete was told where to crack so that it would not crack somewhere you minded. Many of them have already cracked, below the joint, out of sight.

Honour the joint. There are two ways to handle a control joint and only one we recommend, so we are going to say which rather than present it as a coin toss.

• Fill and coat over — what most garage floor companies do, and what we do not recommend. You get an unbroken floor that is easier to clean and reads as one surface. The bet you are making is that the joint will never move again. A slab that is not moving today can move after a winter, as it settles, or under a car parked in the same spot every night — and when it does, the joint telegraphs a fine crack back up through the coating. It is not repairable in place; it reappears.
• Honour the joint — what we recommend, every time. Coat up to it and leave it working, or saw it back open afterwards and fill it with a flexible sealant. The floor keeps a visible line, and the movement has somewhere to go that is not your coating. A fast-curing polyurea joint sealant goes in like caulk and is ready almost immediately, so honouring the joints costs you a line you can see and very little else.

Most residential garage floors are filled and coated over, and most owners would rather have an unbroken floor than insurance against a hairline. Which is right for your slab depends on how wide the joints are, whether they have already moved, and how old the concrete is. Send us the joint width and a photograph and we will tell you what we would do.

The last step is a vacuum

Vacuum the whole floor, then vacuum it again, including the corners and the line where the wall meets the slab. Use a HEPA vacuum. Not a shop vac, and not a shop vac with a fine-dust bag in it — concrete dust is respirable crystalline silica, the particles that matter are the ones you cannot see, and an ordinary filter passes them straight through the motor and back into the room you are standing in. The extractor you hire with the grinder is the right machine; hire it with the grinder.

Do not sweep — a broom moves the coarse dust around and presses the fine fraction into the pores you just spent a day opening. Do not blow the floor out, either: the dust goes into the air and comes back down onto wet coating. Whatever is left in those pores is what the coating bonds to instead of your concrete.

Conditions on the day

Preparation ends the moment the first pail is opened, and the clock after that is short. The base coat’s pot life is 20 minutes at 70°F, and the manufacturer quotes the same span as working time at 75°F in a 3-gallon kit. Either way it is the time from when Part A and Part B are combined, and both heat and a bigger batch shorten it. So the floor is ground, repaired, vacuumed, taped and laid out, with the tools to hand and a second pair of hands agreed, before anything is mixed.

The sheets give a storage range of 50–110°F and an application window of 55°F and above, air at least 5°F above the dew point. Neither is the number that governs your day. The kit has to be at 60–80°F for 48 hours before use — a pail survives the top of the storage range in a shed, but material mixed that warm goes off in the bucket, sometimes fast enough to smoke. The dew point condition matters as much as the temperature: if the slab is cooling towards the dew point it outgasses into wet coating and you get pinholes. The topcoat is the forgiving one here, rated down to 15°F, so a cold garage is more of a problem for the base coat than for the clear. If your garage is cold, or the day is going to be hot, ask us rather than guessing.

The full sequence — mixing, primer, base coat, broadcasting flake, scraping and the clear topcoat — is in the installation hub.