Epoxy Floor Repair: How to Diagnose and Fix a Failing Coating

An epoxy floor rarely fails all at once. It starts with one patch that lifts where the car parks, a few blisters near the door, a crack that has appeared in the coating along a line that was never there before, or a lane that has worn dull. The instinct is to buy something and fix the spot. The better move is to spend twenty minutes working out why that spot failed, because the cause decides whether a repair will hold or come up again next season.

This guide walks through epoxy floor repair in the order that actually works: diagnose the failure, map how far it goes, fix whatever is wrong with the concrete underneath, then repair or recoat the coating. Everything here is drawn from McKinnon Materials' own surface preparation guide and the technical data sheets for our floor coatings. If you are coating a floor for the first time rather than fixing one, start with our garage floor coating guide instead.

The short answer

Most epoxy floor repairs are a four-step job. First, identify what caused the failure, because a patch over an unsolved problem is a patch that fails again. Second, tap the floor to find every area where the coating has let go, not just the part you can see. Third, repair the concrete itself where it is cracked, spalled or weak. Fourth, cut the loose coating back to a sound edge, take the gloss off the surrounding coating so the new material has something to grip, and recoat.

The honest caveat comes first, though: a localized failure with a clear cause is very repairable. A floor that is letting go across wide areas, usually because moisture is coming up through the slab, needs that moisture problem solved before any coating will stay down, and sometimes the right answer is to take the old coating off and start again.

Find out why it failed before you fix anything

The shape and location of a failure tell you a great deal about its cause. Before you sand or buy anything, look at where the damage is and what it looks like.

It lifted in a patch where a tire sits. That is almost always hot tire pickup: a warm tire softens a weak or poorly bonded film and pulls it up as it cools. Our garage floor guide covers the mechanism in detail. The repair question it raises is simple: was the coating bonded well everywhere else, or only just holding on?

It blistered, or it is coming up in sheets. McKinnon's surface preparation guide names water in the concrete as the cause of both. Liquid water in the slab can turn to vapor as the temperature rises, and that pressure raises blisters if the coating has not cured enough to resist it. Water that collects between the concrete and the coating after it has cured causes delamination. The guide lists three sources: water soaked into porous areas during cleaning, water still in concrete that had not finished curing, and ground water moving up through the slab.

It peeled cleanly, and the concrete underneath looks smooth. That points at the bond itself. The coating never gripped the slab, usually because the surface was not opened up, was still carrying an old sealer, oil or curing compound, or was simply dirty when it was coated.

There is a crack running through the coating. Look underneath it. According to the preparation guide, a properly applied coating is strongly bonded to the concrete, so any crack that develops in the concrete will be reflected in the coating. The crack is in the slab, and fixing the coating alone will not stop it reappearing.

The coating is intact, but a lane has gone dull. That is wear, not failure. It is the easiest repair of all and it is covered further down.

Map the damage: the tap test

The part of a failure you can see is usually smaller than the part you cannot. Coating that has let go of the slab can still look perfectly flat. McKinnon's preparation guide describes the way to find it: tap the surface with a hammer, or drag a heavy chain across it. A hollow sound indicates delamination.

Work outward from the damaged area in every direction and mark the edge of the hollow zone with chalk or tape. Then do a quick pass over the rest of the floor, paying attention to the areas that take the most traffic and the areas nearest the door. What you find decides the size of the job. A hollow area the size of a doormat around one tire patch is a spot repair. Hollow areas scattered across the whole floor are a warning that the cause is still active.

Read the back of the piece that came off

If a piece of coating has come up, turn it over before you throw it away. It is the most useful evidence you have.

McKinnon's preparation guide describes how to read a pull-off adhesion test, where a small area of coating is deliberately pulled from the slab, and the same logic works on a piece that has already come up on its own:

• Clean coating with little or no concrete on the back indicates surface contamination. Something on the slab stopped the coating from bonding.
• A thin line of cement and sand stuck to the back indicates weak cement paste at the surface. The coating held onto the top skin of the concrete, and the skin gave way.
• A significant amount of concrete stuck to the back means the coating held and the concrete itself broke. The guide notes that this can indicate the surface was overworked when the slab was finished.

Each answer leads to a different fix. Contamination means the repair area has to be cleaned and opened up far more thoroughly than last time. Weak surface paste means the top skin has to be removed until the concrete underneath is sound. Concrete breaking means the slab itself is the weak point, and the repair starts with the concrete.

Repair it or strip it: how to decide

A spot repair is the right call when three things are true: the failure is confined to an area you can mark out, the coating around it taps solid, and you have identified the cause and dealt with it. A tire patch on an otherwise sound floor is the classic example.

A spot repair is the wrong call when the tap test finds hollow areas all over the floor, when the failure is blistering or sheeting that points at moisture, or when the coating is coming away cleanly almost everywhere you probe it. In those cases the problem is the bond across the whole floor, and patching one area simply moves the next failure somewhere else.

If you suspect moisture, test for it before you spend money. The preparation guide gives the method: tape a two-foot by two-foot sheet of plastic flat to the concrete, sealing the edges, and leave it for 24 hours. If condensed moisture is visible underneath when you lift it, water is moving through the slab. The guide also notes the test is only reliable when the slab temperature is at least 40 degrees Fahrenheit. On a floor that is passing moisture, the coating will keep letting go until that moisture is stopped, which is what a moisture barrier such as MVB Fast Cure is for. On an existing coated floor, that generally means the old coating has to come off where the barrier is going down.

Fix the concrete first

No coating repair outlasts the concrete underneath it. McKinnon's preparation guide covers the three concrete problems you are most likely to find.

Cracks. The guide says cracks larger than a hairline, and any crack that shows signs of structural movement, should be routed out. It describes chipping out the concrete along the crack with a chipping hammer, vacuuming out the debris, and filling the opened channel with a crack compound mixed to its directions. One distinction matters here: cracks that are actually control or expansion joints are a separate subject. Those joints exist because the slab is expected to move, and they are not filled like a crack.

Spalls, holes and voids. Chipped-out areas, porous spots and small voids should be patched with a Portland cement based mortar or a compatible polymeric patching compound, according to the guide. For a larger damaged area, it describes the same approach a professional would use: snap a chalk line around the area, keep it rectangular if you can so the finished patch looks deliberate, saw cut the perimeter, chip out the bad concrete working from inside the cut toward the edge, then trowel in the patching material level with the surrounding floor and let it cure.

Concrete that is not sound. The guide is direct about this: concrete that does not meet a structurally sound condition must be replaced and properly cured before any coating goes back on. If the tap test and the back of the failed piece both point at the concrete breaking, a coating repair is not the fix.

Whatever you patch with, give it the full cure its directions call for before you coat over it.

How to spot-repair an epoxy coating

Once the cause is dealt with and the concrete is sound, the coating repair itself follows a clear sequence.

1. Remove everything that is loose. Scrape or grind away all coating inside the hollow zone you marked, back to an edge where the coating is firmly bonded. A repair laid over coating that has already let go will lift with it.
2. Prepare the bare concrete. The exposed slab needs the same preparation a new floor would get: clean, free of oil and old sealer, and opened up until it feels like rough sandpaper. Our metallic epoxy floor install walkthrough covers slab preparation step by step.
3. Take the gloss off the coating around it. McKinnon's preparation guide says aged epoxy surfaces should have the gloss removed by sanding, screening or shot blasting, and then be wiped with an aggressive, compatible solvent. Feather the sanding a few inches out beyond the repair so the new coat bonds to the old coat at the edge as well as to the concrete.
4. Clean it thoroughly. Vacuum up all the dust. Any film or contamination left on the surface is something the new coat will bond to instead of the floor.
5. Test before you commit. When in doubt about the preparation, the guide recommends installing a test patch of about two feet by two feet, letting it cure, and testing it for adhesion before going further.
6. Recoat, following the data sheet. Apply the same system the floor was built with, in the same order, and follow the mix ratio, recoat window and cure times on the technical data sheet for each product. Those figures differ between products and they are not interchangeable.

One expectation to set honestly: a spot repair is structurally sound long before it is invisible. A solid colour floor usually blends reasonably well. A flake floor or a metallic floor rarely does, because no two pours of a metallic look alike and the pattern cannot be matched. If appearance matters, plan to recoat a whole section, or the whole floor, rather than a single patch.

Can you put epoxy over epoxy?

Yes, provided the existing coating is sound and properly prepared. That is exactly what McKinnon's preparation guide describes for aged epoxy: remove the gloss by sanding, screening or shot blasting, and wipe it down with a compatible solvent. A new coat laid over a glossy surface has nothing to hold onto.

Two further rules come from McKinnon's high performance urethane data sheet, and they are good practice for any recoat. First, test the existing coat before recoating: press on it with your thumb, and if no fingerprint impression is left, recoating can start. Colder temperatures need more cure time before that point. Second, remove any film on the previous coat with a standard detergent cleaner, and degloss it, to ensure a trouble-free bond.

Compatibility matters as much as preparation. A coating bonds to what it touches, so over an incompatible film it inherits that film's problems. As one example, McKinnon's own documentation notes that our concrete sealer will not adhere satisfactorily to epoxies or polyurethanes, which is why a paver or concrete sealer is not a repair product for a coated floor. If you do not know what is already on your floor, a small test area tells you before a whole floor does.

Bubbles, blisters and craters

Not every defect is a bond failure. Some happen while the coating is still curing.

Blisters from the slab. The preparation guide explains that air trapped in porous areas of the concrete can expand before the coating cures, and that the same vapor pressure from damp concrete can raise blisters as the temperature climbs. Its advice for controlling them on the next coat: seal the surface first by applying a heavy primer coat and letting it at least partially cure before continuing, and choose conditions that allow the coating to cure before the temperature rises.

Craters and pinholes from outgassing. McKinnon's metallic system instructions warn that outgassing leaves noticeable craters in the floor that are not easy to repair and blend in, which is why the primer coat and the timing of the pour matter so much. Our garage floor guide covers the practical rule: coat while the slab temperature is falling, not rising in the morning sun.

For craters and pinholes already cured into a floor, the fix is a recoat done properly: sand the area to remove the gloss, clean it, and apply the next coat so it fills the defects, following the recoat guidance above.

When a whole-floor recoat is the honest answer

Sometimes nothing is broken at all. After years of traffic, the lanes where people and cars actually travel go dull while the rest of the floor still shines. That is a worn topcoat, and the fix is a fresh topcoat over the whole floor, not a series of patches. Our guide to cleaning an epoxy floor explains how to tell wear from dirt and how to refresh the topcoat when cleaning is no longer enough.

A full recoat also makes sense when a floor has several scattered repairs that will never blend, or when you want to change the finish. McKinnon's topcoats include Polyaspartic Clear Floor Coating and 2-Part Water-Based Urethane, which comes in gloss or matte. The whole range is in our industrial floor epoxy system collection.

How to avoid the next repair

Almost every failure described above traces back to the same few decisions made before the first coat went down. The preparation guide says it plainly: improper preparation is the number one reason for job failure. When you repair, fix the process that caused the problem as well as the floor:

• Test for moisture before coating, every time, and use a moisture barrier if the test shows it.
• Open up the concrete properly and remove every old sealer, oil stain and curing compound.
• Repair cracks and spalls before coating, not after.
• Respect the recoat window. Missing it turns a chemical bond into a sanding job.
• Coat when the slab temperature is falling, and let each coat cure fully before traffic, especially before a car parks on it.

If you are unsure whether your floor is a repair or a strip-and-recoat, call us at 888-849-0588 before you buy anything. Tell us what failed, where, and what the back of the loose coating looks like, and we will help you work out the right next step.

Frequently asked questions

Can you repair a peeling epoxy floor?

Usually, yes, if the peeling is confined to an area you can mark out and the coating around it is still firmly bonded. Remove the loose coating back to a sound edge, fix whatever caused the failure, prepare the bare concrete, degloss the surrounding coating, and recoat with the same system. If the peeling is widespread, the bond is failing across the whole floor and a patch will not hold.

Is there an epoxy floor repair kit?

A repair is done with the same materials the floor was built with, plus a suitable patching compound for any damaged concrete. McKinnon does not sell a separate repair kit. The coating itself should match the system already on the floor, applied following that product's technical data sheet.

Can you put new epoxy over old epoxy?

Yes, if the old coating is sound. Remove its gloss by sanding, screening or shot blasting, clean it thoroughly, and wipe it with a compatible solvent before recoating. New epoxy over a glossy, dirty or loose coating will not bond properly.

How do I know if my epoxy coating is coming loose?

Tap the floor with a hammer or drag a heavy chain across it. A hollow sound indicates the coating has let go of the concrete underneath, even where it still looks flat. Mark the hollow areas so you know the real size of the repair.

Why is my epoxy floor blistering?

Blisters are most often caused by moisture or trapped air in the concrete. As the temperature rises, water vapor or air in the slab expands and pushes up a coating that has not yet cured enough to resist it. Test the slab for moisture with a sealed sheet of plastic left for 24 hours before recoating.

Should I fix the cracks in the concrete before recoating?

Yes. A well bonded coating reflects any crack that develops in the concrete beneath it, so a crack that is not repaired will show through the new coat. Cracks larger than a hairline are routed out and filled with a crack compound. Control and expansion joints are a separate case and are not filled like cracks.

Will a spot repair on an epoxy floor be visible?

Often, yes. A solid colour floor can blend reasonably well, but flake and metallic floors rarely match because the pattern cannot be reproduced. If the look matters, recoat a full section or the whole floor so the finish is uniform.

Need help deciding? Call McKinnon Materials at 888-849-0588, Monday to Friday, 8:30 AM to 5:00 PM ET, or browse our floor coating system.