How long the room is out of use

The schedule, hour by hour

Everything below is the same job at your slab temperature. Note where the long gaps are: they are between coats and after the final one, not during the work.

The clock that starts before the one on this page

Everything above assumes the slab is ready to be coated. On a new pour it is not, and that wait is measured in months rather than hours. The federal painting specification puts a number on it, and the number it gives a floor is not the number it gives a wall.

Curing: Allow concrete, stucco and masonry surfaces to cure at least 30 days before painting, and concrete slab on grade to cure at least 90 days before painting.

What that changes for your floor A slab on grade can only dry upward, because the ground beneath it is not going anywhere, and that is the whole reason one paragraph gives concrete generally thirty days and a slab on grade 90 days. On concrete poured this spring, the schedule above does not start counting until late summer, and neither heat nor fans move that date forward by much.

Two limits on that rule are worth stating plainly. It is a construction specification written for federal work rather than a product instruction, so it does not know your resin and cannot override a data sheet that asks for longer. And it is a calendar standing in for a measurement: ninety days describes a slab drying in ordinary conditions, and it is not evidence about your slab. A floor of that age under a leaking roof through a wet autumn can still fail a quantified moisture test, and the quantified test is the one that actually governs — that is what the moisture page is for.

The rule also stops applying in one direction. Products sold as moisture-vapour barriers exist precisely so that a green slab can be coated, and where one is specified the requirement that governs is the one printed on that product rather than the calendar in a painting specification. Buying mitigation to recover several weeks of waiting is a real trade with a real price, and it belongs in the installed cost rather than in this schedule.

The temperature rule, stated plainly because it is an approximation

This page stretches every published window by a factor of two for each 18 Fahrenheit degrees your slab is below the data sheet's reference temperature, and shrinks it by the same factor when the slab is warmer. At the settings above the multiplier is 2.08.

That eighteen-degree figure is the Fahrenheit form of the familiar ten-degree-Celsius reaction-rate rule of thumb. It is a rough approximation for a resin system and it is not your product's chemistry. It is here because it is far better than assuming the data sheet numbers apply unchanged in a cold garage, and far worse than a data sheet that publishes a second temperature column. If yours does, use that instead of this.

The recoat window is the thing that catches people out

There are two recoat times on a data sheet, not one. Before the first, the previous coat is still soft and the new one will wrinkle it. After the second, the previous coat has cured hard enough that the new one bonds mechanically rather than chemically — which means it has to be abraded first, turning a second coat into a second preparation.

Cold weather stretches both ends, so the window moves later in the day rather than getting wider, and a two-coat job planned for a Saturday quietly becomes a Saturday and a Sunday morning. The number that matters is the gap between them: 24.9 hours at these settings.

What cold actually does, beyond making you wait

• Viscosity climbs, so the material does not level and roller marks stay in the film.
• Below the mid fifties many systems stop curing rather than curing slowly, and some never reach full hardness afterwards.
• A cold slab under warm humid air condenses moisture onto its own surface, which is a bond breaker you cannot see.
• Heating the space warms the air long before it warms the slab, so an air thermometer reads success while the concrete is still cold.

If you have to work cold, heat the slab for a day beforehand rather than heating the air on the morning, and check the concrete with an infrared thermometer rather than trusting the room.

Consequences of working at this temperature