Can You Epoxy a Garage Floor With Cracks? Crack Repair, Moisture Testing and Prep for Spokane Garage and Shop Floors (2026)

Yes, you can epoxy a garage floor that has cracks, but only after each crack is sorted into "moving" or "not moving" and repaired the right way, and after the slab passes a moisture test. Coatings follow the concrete underneath them. A crack that's still moving will come right back through a new epoxy or polyaspartic floor, and a damp slab can push the coating off no matter how well the cracks were filled.

Here's how the coating manufacturers' own data sheets and prep guides say to handle cracks, joints, moisture and surface profile before a garage or shop floor gets coated in the Spokane area.

Short answers

• Can you epoxy over cracks? Yes, if they're repaired first. ONYX Coatings' data sheets say control joints and random cracks "should be saw cut or chased first, then filled with the appropriate patch material"
• Can you just roll epoxy over a hairline crack? Sometimes. Sherwin-Williams High Performance Flooring says shallow craze cracking in the surface "usually does not require any special treatment and is usually mitigated in the prime coat." Deeper cracks need to be opened up and filled
• What about cracks that move? Treat them like joints. Sherwin-Williams says settling cracks "should be treated as moving joints and honored through the system," and to call a structural engineer if there's known movement or visible heaving
• Do I have to test for moisture? Every data sheet we checked asks for it, using ASTM F1869 (calcium chloride) or ASTM F2170 (in-slab relative humidity)
• Can I acid etch instead of grinding? Most systems want mechanical prep. Arizona Polymer Flooring's Epoxy 600 data sheet says acid etching "is not recommended and will void manufacturers' warranty"

Step 1: Figure out what kind of crack you have

Sherwin-Williams' crack treatment guidelines split cracks into two groups:

• Dynamic (active) cracks: the cause is still at work and the crack is still moving. Examples are settling or structural cracks from a sub-base that shifted, heaving cracks and cracks from ground movement
• Static (dormant) cracks: cracks that aren't likely to become active, or whose movement is too small to affect a repair. Examples are craze cracks and plastic shrinkage cracks from the slab's early curing

Wolverine Coatings' technical bulletin adds a useful caution: "it is good practice to assume movement in a joint or crack rather than none," because all cracks and joints move a little when the concrete's temperature changes. That matters in an unheated Spokane garage, where the slab swings with the seasons.

Signs a crack may still be moving:

• One side sits higher than the other (heave or settlement)
• The crack has gotten wider since you first noticed it
• It runs through the slab and lines up with a soft spot, a downspout or a spot where water collects

If that sounds like your floor, the slab problem comes before the coating. Our driveway cracking guide covers what causes cracks around Spokane and which ones to worry about.

Step 2: Repair static cracks so they don't telegraph

A repaired crack disappears under the coating, so it can't be re-cut later. Wolverine Coatings says that's why cracks "must be properly sealed before installing a coating system to minimize cracks 'telegraphing' through the coating." The manufacturers' prep guides line up on the method:

• Open the crack up: Wolverine says to widen cracks "at least 3 times their width or 3/8" minimum, preferably with a 'v' notch." Sherwin-Williams calls for a sawcut with a V-notch blade on a vacuum, after removing laitance, debris and sealers
• Fill with a semi-rigid filler: Wolverine recommends semi-rigid joint filler because it takes "some movement" but carries more load than a highly flexible sealant. For epoxy systems, Sherwin-Williams lists a thickened epoxy filler for cracks under 1/4 inch wide and an epoxy mixed with its screed aggregate for wider cracks
• Reinforce if needed: Sherwin-Williams lists an optional fiberglass mesh at least 6 inches wide over static cracks, and says plastic shrinkage cracks can be bridged with a mesh-reinforced system
• Grind it flush: we grind the cured filler level with the slab during surface prep, so it doesn't show as a ridge in the finish

Step 3: Handle joints differently from cracks

Joints are planned breaks in the slab, and the prep guides treat them by type:

• Control (contraction) joints: the saw cuts that tell the slab where to crack as it shrinks. Sherwin-Williams and Dur-A-Flex call these "non-moving" joints and fill them with a semi-rigid filler after cleaning them out to about 3/4 inch deep and 1/4 inch wide. ONYX says the same: chase control joints and fill them before coating
• Construction, expansion and isolation joints: these are expected to move. Dur-A-Flex recommends "honoring" them by saw cutting back through the finished floor, at least 3/4 inch deep and 1/4 inch wide, adding a closed-cell backer rod and filling with a flexible joint sealant. ONYX's data sheets say to saw cut through the cured coating over construction joints
• Why it matters in a cold garage: Dur-A-Flex warns that if the floor's temperature changes 20 degrees or more between when a joint is filled and coated and its normal operating temperature, hairline cracking can show up even over non-moving joints
• Young slabs: Sherwin-Williams and Dur-A-Flex both note that the American Concrete Institute recommends deferring joint filling in industrial floors 60 to 90 days after the pour, or as long as possible, so the joints can open up through shrinkage first

Honoring a joint leaves a visible line in the finished floor. Wolverine calls honoring "the safest way to guard a coating against cracking due to movement," but says it may not meet everyone's look. That's a trade-off to decide before the job starts, not after. Our control joint spacing guide explains where those joints should be in the first place.

Step 4: Test the slab for moisture

Moisture coming up through the slab is a common reason coatings blister and let go. ONYX says "if vapor is above this level, then blistering and delamination of the coating may occur." These are the two standard tests:

• ASTM F1869 (calcium chloride): a dish of calcium chloride is sealed under a dome on bare, cleaned concrete. Gilson's kit manual calls for the space to be held at 65 to 85°F and 40 to 60% relative humidity for at least 48 hours first, then 60 to 72 hours undisturbed. Results come out in pounds per 1,000 sq ft per 24 hours
• ASTM F2170 (in-slab relative humidity): probes go into holes drilled to 40% of the slab's thickness for a slab drying from the top only, or 20% if it dries from both sides. Wagner Meters' example: a 6-inch slab on grade gets holes 2.4 inches deep
• How many tests: Gilson's F1869 manual calls for three test kits for areas up to 1,000 sq ft and one more for each additional 1,000 sq ft, so even a two-car garage gets three

What the data sheets accept:

• ONYX Polyaspartic 90: 3 lb per 1,000 sq ft per 24 hours by ASTM F1869
• Leggari Polyaspartic: 3 lb by F1869 or 80% RH by F2170
• Chemtec CHEM1000 PG polyaspartic: 3 lb by F1869 or under 85% RH by F2170
• APF Epoxy 600: F2170 testing required, with a moisture-mitigation system if readings exceed 85%

ONYX, Chemtec and APF all call for a vapor-control or moisture-mitigation system, not coating anyway, when the slab is over the limit, and Leggari says to consult its representative or installation guidelines. Leggari adds that passing the test "is not a guarantee against future problems," especially if there's no working vapor barrier under the slab.

The Spokane catch: the calcium chloride test's 65 to 85°F conditions won't happen in an unheated garage in winter, so the space has to be heated first or the test waits. Our winter epoxy guide covers slab temperature and dew point, and our vapor barrier guide explains why what's under the slab matters.

Step 5: Profile the surface (grind, don't etch)

A coating needs texture to grip. The International Concrete Repair Institute's guideline ICRI 310.2R defines 10 concrete surface profiles, from CSP 1 (nearly flat) to CSP 10 (very rough), and coating data sheets name the profile they need:

• APF Epoxy 600: concrete at least 30 days old, clean and sound, mechanically prepared to a minimum of CSP 2 to 3, vacuumed, and no acid etching
• Chemtec CHEM1000 PG: CSP 2 and above depending on the system, by shot blasting, grinding or another mechanical method, with cracks and chips repaired first
• ONYX: "Mechanical methods are required for preparing concrete prior to coating application." Its Polyaspartic 90 data sheet also says polyurethanes and polyaspartics in its line "should not be applied directly to concrete," and go over an epoxy primer or broadcast base instead

Grinding also brings filled cracks and joints flush with the slab. If your floor has an old sealer, paint or a curing compound on it, that has to come off before the moisture test and the coating.

Your crack-and-prep checklist

• Cracks sorted into moving and static, with anything heaving or settling looked at first
• Static cracks opened up and filled with a semi-rigid filler, then ground flush
• Control joints cleaned out and filled; construction, expansion and isolation joints marked to be honored
• Moisture tested by ASTM F1869 or F2170, with the right number of tests for the floor's size
• Slab mechanically ground or shot blasted to the profile on the product's data sheet
• Slab at least as old as the data sheet requires (APF says 30 days)
• Product data sheet in hand, so the filler, primer and topcoat all match. Our epoxy vs. polyaspartic guide compares the two systems

When the slab needs more than crack repair

Coatings can't fix a slab that's settling, heaving or breaking up. If sections have sunk, rocked or shattered, those sections need to be repaired or replaced before any coating goes down. Raven pours garage floors, shop slabs and replacement sections, so we can tell you whether your floor is a coating job or a concrete job first. See our concrete flatwork services.

Get a garage or shop floor quote

Every floor is priced after we see the cracks, the joints and the moisture readings, so we quote each one from a site visit. Call 509-350-2016 or request a free quote with your address, the floor's size and a few photos of the worst cracks.

Sources

• Sherwin-Williams High Performance Flooring: Crack Treatment Guidelines (PDF)
• Sherwin-Williams High Performance Flooring: Joint Guidelines (PDF)
• Dur-A-Flex: Joint Guidelines (PDF)
• Wolverine Coatings: Treatment of Cracks and Joints in Concrete, Technical Information Bulletin (PDF)
• ONYX Coatings: Polyaspartic 90 Technical Data Sheet
• Leggari: Polyaspartic Technical Data Sheet (PDF)
• Chemtec Coatings: CHEM1000 PG Polyaspartic Technical Data Sheet (PDF)
• Arizona Polymer Flooring: APF Epoxy 600 Product Data Sheet (PDF)
• Gilson: Moisture Emission Test Kits Manual, ASTM F1869 (PDF)
• Wagner Meters: Performing Correct and Accurate RH Testing in Concrete, ASTM F2170 (PDF)
• ICRI 310.2R-2013: Selecting and Specifying Concrete Surface Preparation for Sealers, Coatings, Polymer Overlays, and Concrete Repair