Garage Floor Coatings in Lowcountry Humidity

A coated garage floor looks great the day it goes down. The ones I see peeling two summers later almost never failed because of the product. They failed because of what was coming up through the concrete, or because nobody prepared the surface the way the manufacturer said to.

The Moisture You Can’t See

Concrete breathes. Moisture moves up through a slab from the ground, and in a humid, low-lying place like this there’s plenty of it. The American Concrete Institute’s guidance on moisture-sensitive floors includes epoxy coatings and lists the problems moisture causes: blistering, delamination, adhesive degradation and mold growth. (ACI PRC-302.2-22) Sika, one of the big coating makers, citing industry groups, says moisture-related floor failures account for more than 90 percent of all flooring claims. (Sika)

So the first question for any installer is whether they test the slab. There are two standard tests. The calcium chloride test (ASTM F1869) measures how much moisture comes out of the surface, with the test dish sealed in place for 60 to 72 hours. The in-slab relative humidity test (ASTM F2170) puts probes into the concrete. (KTA-Tator) Product data sheets set limits from those tests. One Sika epoxy, for example, calls for slab humidity at or below 85 percent; wetter slabs call for a different product. (Sikafloor-264)

Prep Is the Job

Sherwin-Williams says it plainly: “Most coating manufacturers no longer recommend acid etching.” It leaves too little profile for most systems and puts water into the slab. Contractors grind or shot-blast instead. (Sherwin-Williams) Sika’s data sheet calls for mechanical prep to a specific roughness, a profile of CSP 3 to 4 on the industry scale. A box-store kit with an acid etch packet is a different animal.

Weather matters too, and here it matters a lot. The data sheets I read both set a 5-degree Fahrenheit margin over the dew point: Sika applies it to the slab, and the polyaspartic maker applies it to the air. One polyaspartic says not to apply it over 80 percent humidity, and a Sika epoxy caps it at 85 percent during application and cure. (APF Polyaspartic 7500, Sikafloor-264) On a muggy August morning in a closed garage, that’s not a small detail. Ask how they check it.

Epoxy, Polyaspartic or Polyurea

• Epoxy is the old standard. Its weakness is sunlight: Sika’s data sheet warns it “will discolor over time when exposed to sunlight (UV).” In a garage with the door open half the day, that shows.
• Polyaspartic is “UV stable” per the manufacturer data sheet I read, cures fast enough for foot traffic in about 4 to 6 hours and cars in about a day, and goes over a primer, not straight onto concrete. (APF)
• Polyurea is more flexible and cures quickly. Its makers say so, and they’re not neutral, and some polyureas can also yellow in sunlight. (ArmorThane)

A lot of jobs are a layered system, with a base coat, decorative flake and a topcoat. Ask what each layer is made of, and whether the top one is rated for sunlight.

One more failure you’ll hear about is hot tire pickup, where hot tires soften the coating and pull it off the floor. Moisture pushing up from below makes it worse, and so does prep that was “simply cleaning or acid etching.” (Polycote)

What It Costs

National figures, since I found no local ones: This Old House puts professional epoxy at $3 to $12 per square foot, with DIY at $2 to $5. (This Old House, March 2026) Bob Vila puts polyaspartic at $4 to $9 per square foot, or about $1,600 to $3,600 for a 400-square-foot garage. (Bob Vila, January 2024)

The Floor You Can’t See

Most of a good coating job is work nobody will ever look at: the moisture test, the grinding, the crack repair and checking the dew point before anything gets mixed. When you compare quotes, compare that part. The shiny part is the same everywhere.