Two things decide whether a coating stays on, and neither is the product
Coating failures on garage floors almost always come down to one of two causes, and both are decided before a tin is opened. The first is adhesion, which depends on the surface having enough profile for the coating to key into. A power-trowelled slab is effectively polished. Nothing grips a polished slab reliably, and the fix is mechanical or chemical preparation that opens the surface until it feels like sandpaper rather than like a worktop.
The second is moisture. Concrete on the ground is in contact with whatever is under it, and unless there is an effective barrier below the slab, water vapour moves upward through it continuously. That is invisible on bare concrete because the moisture evaporates off the surface. Put a coating over it and the coating becomes the barrier, the vapour accumulates underneath, and the pressure lifts it away from the slab in sheets. This does not happen on the first day. It happens after weeks or months, which is long enough that people blame the product.
The plastic sheet test
Tape a square of clear plastic sheet flat to the slab, sealed on all four edges, and leave it in place. Then look. Condensation on the underside of the plastic, or a patch of concrete that has visibly darkened, means moisture is coming through. A dry sheet and a dry slab is the result you want.
Do it in several places rather than one, because slabs are not uniform. Test near an outside wall, test the coldest corner, test anywhere the floor is below the outside ground level, and test the middle. It is worth remembering that this is an indication rather than a certificate: a slab that tests dry at the end of a hot summer can behave differently after a wet winter, and a floor that has been covered by boxes for years has not been evaporating freely and may need time uncovered before the test means anything.
If the test fails, the question is why, and that is about ground water, site drainage and whether a barrier exists under the concrete. Those are not questions a coating answers. Spending more on the coating makes the failure more expensive, not less likely.
Why published coverage is optimistic on a real slab
Coverage figures come from applying a known film thickness to a smooth panel. A slab prepared properly has texture, and texture is extra surface area plus a lot of small voids that the first coat sinks into. The rougher the preparation, the more the first coats disappear.
| Surface after preparation | Feel | Rough allowance on the first coats |
|---|---|---|
| Power trowelled, untouched | Smooth, slightly slick | Published figure, and poor adhesion |
| Acid etched or lightly abraded | Fine grit | Around 10 to 15% more material |
| Ground or lightly shot blasted | Medium sandpaper | Around 25% more material |
| Heavily blasted or scarified | Open, visibly porous | Around 40% more, sometimes far more |
Those are planning allowances for estimating, not specifications. The reason coverage is an input on this page rather than a built-in constant is that the real figure comes off the data sheet for the product being used, at the film thickness being applied, and it varies enormously between a thin roll-on floor paint and a self-levelling build coat. The calculator applies the profile allowance in full to the prime coat, at half strength to the first build coat, and not at all after that, on the basis that by the third pass you are rolling onto film rather than into concrete.
The parts of the job that are not volume
Mixed two-part material has a working time, and it shortens as the temperature rises. That is why the sensible unit of work is a kit at a time and why mixing ahead is a way of converting product into a bucket of solid waste. It is also why the surplus figure matters: the difference between the volume needed and the kits bought is not spare material to keep, because once parts A and B meet, the clock is running.
Ventilation and eye protection are the handling points worth stating plainly. Two-part and solvent-borne coatings put vapour into the air while they are mixed and applied, some epoxy hardeners and some polyurethane topcoats are respiratory sensitizers where a reaction develops over repeated exposures rather than on first contact, and a garage attached to a house shares air with the house. The safety data sheet for the specific product governs what is needed. This page names the hazard and does not choose equipment for you.
If the slab itself is new rather than existing, its age matters as well as its moisture, and if it is cracked or spalling the repair comes first. Volume for a new pour is worked out in the concrete slab calculator. Once the floor is done, the shelving that goes around it is easier to plan before the units are standing on the new surface — that is the garage shelving planner — and the bay clearances that decide where anything can stand are in the garage vehicle fit calculator. For coating walls rather than floors, the arithmetic is different enough to belong in the paint calculator.
Questions people ask
How do I test a garage floor for moisture before coating it?
Tape a square of clear plastic sheet flat to the slab, sealed on all four edges so no air gets under it, and leave it in place. Then lift a corner and look. Condensation on the underside of the plastic, or concrete that has visibly darkened where the sheet was, means vapour is moving up through the slab. Do it in several locations rather than one: near an outside wall, in the coldest corner, anywhere the floor sits below the outside ground level, and in the middle. A dry result in every location is what you want before buying material. It is an indication rather than a guarantee, since slabs behave differently by season, but it is the cheapest useful step in the whole job.
What happens if I coat a slab that fails the moisture test?
The coating becomes a barrier that the moisture cannot get through, so it accumulates underneath and eventually lifts the film off the concrete. It typically comes away in sheets rather than flaking, and it typically happens weeks or months later rather than immediately, which is why the product usually gets the blame. More coats do not help, because more coats is a better barrier. A more expensive product does not help either. The problem is underneath the slab and the answer, if there is one, is about ground water, drainage and whether a vapour barrier exists below the concrete, which is a conversation to have before any material is bought.
Why does the calculator ask for coverage instead of knowing it?
Because it varies by more than a factor of two between products, and using a built-in number would be inventing a figure for a specific tin the calculator has never seen. A thin roll-on floor paint might publish 400 or 500 square feet per gallon, a build coat at a proper film thickness might be 200 to 250, and a self-levelling application far less again. Those figures are on the data sheet along with the film thickness they assume. What the calculator adds is the part the data sheet does not: the allowance for the profile you have prepared, the waste and cut-in allowance, and the rounding up to whole kits.
Do I need to grind the floor, or is etching enough?
That is a product question rather than a general one, and the data sheet for the coating will state the surface condition it expects. What can be said generally is that the coating has to key into something, that a power-trowelled slab is too smooth to key into, and that the test people use is whether the prepared surface feels like sandpaper. Etching opens the surface chemically and is less aggressive; mechanical grinding or blasting produces more profile and removes contamination and old sealer along with it, which etching does not. More profile means better adhesion and noticeably more material used on the first coats, which is the trade this calculator prices for you.
How much extra should I buy?
The waste allowance covers cut-in at the walls, what stays in the tray and on the roller and what goes off before it can be spread, and ten percent is a reasonable starting point that gets less generous as the floor gets more broken up by cabinets and obstacles. Beyond that, the rounding to whole kits usually provides a margin, and the calculator shows what that surplus is. What it is not is material to save for later: once the two parts of a kit are mixed the working time is running, so surplus in the tin is fine and surplus in the bucket is waste. Buying one extra kit as insurance is a judgement about whether running out mid-coat, with a wet edge going off, is worse than an unopened kit on the shelf.