Spec notes · 2026-09-11

Garage floor epoxy prep on an older Cape Girardeau slab, and why we grind it

Most of the garage floors we coat around Cape Girardeau and Jackson were poured between the 1970s and the 1990s under ranch houses, and they sit directly on the ground. On a garage floor epoxy job the coating goes on day two. Day one belongs to that slab, and what the crew finds on day one decides how the floor turns out. This is what happens on day one, in order, and what each step turns up on an older slab here.

What does the moisture number on an old garage slab mean?

It is a measure of how much water vapor the slab is pushing up through its surface, and it sorts the floor into one of three outcomes. The reading is taken at the measure, so it is in hand before the prep crew arrives, and it goes on the written estimate.

• Under the limit for the system. The standard stack goes on, primer through top coat.
• Over the limit, but not by much. A vapor-mitigation primer replaces the standard primer. It is a moisture-tolerant epoxy that goes down first and holds the vapor under it. It moves the estimate, and it is the only line that changes.
• Well over. We tell you to wait for the season. A slab that reads high in April is reading the ground under it in April.

That last outcome comes up more here than in drier parts of the state, and the reason is under the concrete. An on-grade slab from that era has whatever the builder put beneath it, which around here was often gravel and maybe a sheet of plastic. Below that is Southeast Missouri clay. In a wet spring, when the Mississippi is up and the ground has been saturated since February, that clay holds water against the underside of the slab and the concrete draws it upward like a paper towel. The same slab in late summer, after the ground has drained, can read fine. A high spring reading means we test again in August.

There is a spot we check first on nearly every older garage in Cape and Jackson. The strip of slab inside the overhead door is a shade darker than the rest, even after weeks without rain. Decades of rain blowing under the seal, plus an apron that settled toward the garage and drains the wrong way, have kept that band wetter than the field. It is where the highest reading comes from, and it is where a coating would blister first if the number were ignored.

Summer adds a different moisture problem, on the surface rather than in the slab. A garage floor cools overnight. Open the door on a July morning here, when the air is thick enough to fog your glasses, and that warm wet air condenses on cool concrete. The slab sweats, and nothing bonds to a sweating slab. So on coating morning the crew reads slab temperature against the dew point, keeps the door down, and runs air until the concrete is warmer than the point where the air can condense on it. A floor coated over a film of dew fails the same way a floor coated over a wet slab does.

Why grind a garage floor instead of acid etching it?

Because of what is on an old slab that acid does not remove. A diamond grind cuts the top skin off the concrete and leaves a profile the primer can lock into, CSP-2 on the scale coating manufacturers use. Acid etch dissolves a little cement paste and leaves a slight tooth on clean, bare concrete. On a garage floor that age there is no clean, bare concrete. An etch leaves all of this in place.

• Laitance, the weak chalky skin of cement paste and fines that floated to the top when the slab was troweled. It is the layer that dusts on your broom. Etching roughens it, and a coating bonded to laitance is bonded to dust. The grinder takes it off and reaches sound concrete.
• Old sealers. Some slabs got a cure-seal at the pour, and plenty more got a garage floor sealer rolled on by an owner years ago, which is usually why water beads on the floor. Acid does not touch either one. A grinder does, and you can watch it happen, because the first pass comes off gummy and gray wherever the sealer was.
• Oil. The tire path and the spot under the engine have decades of drips soaked into the pores. Grinding opens the concrete, we degrease the dark spots, and we grind again until the dust comes up dry and pale. Coat over an oil-soaked spot and the oil comes back up through the coating.
• The etch itself. Acid leaves its own salts in the slab and puts water into concrete you have already measured for moisture. Now the floor has to dry again, which in a humid Missouri summer is not a same-day event.

The grind runs with a dust extractor on the machine, so the house does not fill with concrete dust. It also shows you things about the slab. Where the grinder skates, there is sealer. Where the dust comes up dark, there is oil. Edges and the area around the door tracks get done by hand, and then the whole floor is vacuumed down to bare concrete.

What gets repaired before the primer goes on?

Once the slab is ground, everything that was hiding under paint and grime is visible, and all of it gets fixed before the primer, because a primer does not bridge anything.

On an older garage here the repair list is short and predictable, and roughly when the house went up tells us most of it before anyone opens the door. Usually a crack down the middle, because many of these slabs were poured without control joints and the concrete found its own place to break in the first winter. That gets chased open with a blade, filled with a repair material chosen for whether the crack is still moving, and ground flush. If one side of your crack sits higher than the other, the two halves have moved against each other, and that is the reading that picks the filler. Spalls along the door line, where the slab meets the apron and has taken road salt and freeze-thaw for decades until the top skin popped in patches, get cut back to solid concrete and rebuilt with repair mortar. Pitting in the tire path is filled and ground flush, and the flake field hides it completely.

If the slab already has a coating on it, day one starts with taking it off. Whether that floor was ever salvageable is a separate question, and the repair article sorts it out. Either way, removal is priced as prep.

Why is the prep day most of the price?

Because the hours are on day one. The resin for a two-car garage is a few buckets, and it goes down in a day. Day one is a planetary grinder, diamond tooling that wears down per floor, a vacuum, the moisture test, the crack and spall materials, and a crew on its knees at the edges for most of a day. That is why the ranges on our cost page, $6-9 per square foot for a full-flake system and between $2,400 and $3,600 for a two-car garage, already include diamond grinding and standard crack and spall repair. The number is mostly day one.

The one item that moves it is the mitigation primer, and the moisture test tells you whether it applies before you sign anything. The cost page has the full table by system.

It is also why quotes for the same garage can be far apart. A quote with no grinding and no moisture reading is cheaper because day one was left out of it. Ask a low bid how the slab gets profiled and whether anyone metered it. Ours comes back in writing, with the primer named on it and the moisture reading that decided it.