Epoxy Recoat Windows
Every recoat window is printed at a temperature, and the slab is almost never at it. This is how to read the window off the sheet, move it for the slab in front of you, and arrange the gap between coats so it lands inside.
Read the earliest and latest recoat off the data sheet together with the temperature they are quoted at. Then move both edges for the slab: cure speed roughly doubles for every 10°C (18°F) of warmth and halves for every 10°C of cold. Take the slab temperature through the whole wait, not the warmest reading of the day, and plan the gap between coats so it lands inside the window you worked out, not the one printed on the sheet.
A recoat window fails at two edges, and both are covered elsewhere on this site. Recoating before it opens is how a floor wrinkles, and recoating after it closes without abrading is how a top coat ends up in intercoat delamination. This page is about the step before either: working out where the window really is on the job in front of you.
What the recoat line on a data sheet tells you
Most sheets give the window as two times in a timing table, close to pot life and the cure stages. How to read a technical data sheet covers where that block sits. Within it, three things decide what the window means.
The temperature
- Every window is quoted at one, usually as a footnote
- 70°F and 77°F are the most common; check which
- A humidity is often stated alongside it
- No temperature means the window cannot be moved; ask the maker
The maximum
- Sometimes printed as “recoat within” or “or abrade”
- Some sheets state none; confirm that rather than assume it
- Past it, the sheet says how to prepare, and that instruction is part of the warranty
A table by temperature
- Some sheets give the window at several temperatures
- A measured figure for your temperature beats any rule
- Between two rows, scale from the nearer one
Watch the units as well. Fast chemistries are often given in minutes and slow epoxies in hours, sometimes on the same sheet for different coats. And read the sheet for the product you are actually using: fast, slow and low temperature versions of the same product carry different windows.
Scaling the window for the slab
Take an illustration sheet, not a real product, that gives a window of 12 to 24 hours at 77°F (25°C). The crew lays a coat in the afternoon and comes back the next morning, 16 hours later. Here is the same sheet on three slabs.
| Slab | Window opens | Window closes | Next morning (16 hr) |
|---|---|---|---|
| 59°F (15°C), cold slab | 24 hr | 48 hr | Too soon, opens in 8 hr |
| 77°F (25°C), at the sheet temperature | 12 hr | 24 hr | Inside, 8 hr left |
| 95°F (35°C), hot slab | 6 hr | 12 hr | Too late, closed 4 hr before |
Down the side is the slab temperature through the wait. Across the top are the two edges of the window, in hours after the last coat, and what a check 16 hours later finds. The multipliers are 2 (twice as long) on the cold slab, 1 (as printed) at the sheet temperature and 0.5 (half as long) on the hot one, from the rule that cure speed doubles for every 10°C.
The point of the table is the last column. The next morning sits comfortably in the middle of the printed window, which is why it feels safe, and on two of the three slabs it is wrong in opposite directions. Nothing about the floor looks different in the morning either way.
Scale the window on your own sheet
Enter the recoat window from your data sheet, the temperature it is quoted at and the slab temperature. Add the hours since the last coat for a verdict.
Returns a window of 24 hr to 48 hr, so a check 16 hours later is too soon
Open the live calculator →Which temperature to use
The slab, because that is what the coat is curing at. The air can be warm while the slab is still cold, since concrete follows the air slowly; applying epoxy in cold weather covers why it lags by hours. Take it with an infrared thermometer.
Then use the temperature through the wait, not the one at the moment you finish. A slab coated in the warm part of the afternoon cools overnight, so the reading when the crew leaves overstates how fast the coat will cure before it comes back. When in doubt, use the coolest the slab will reach during the wait. On a cold slab that moves the window later, which is the safe direction for the opening edge. On a hot slab, check the closing edge against the warmest the slab gets, because that is what closes it early.
Planning the gap between coats
Work out the window before the first coat goes down, not when the crew arrives for the second. That turns the window into a schedule decision while there is still room to change it.
- Cold slab, window opens late. Either plan a longer gap, or warm the space and the slab ahead of time and hold it there through the wait. Never heat the surface of the coat directly, which is the fastest way to the wrinkle you were trying to avoid
- Hot slab, window closes early. The overnight gap may simply not fit. Either recoat the same day once the window opens, choose a product with a longer window, or plan to abrade before the next coat and price that time into the job
- Temperature changing through the day. Plan on the temperature the slab will be at for most of the wait, and check the edge that matters against the extreme: the coolest for the opening, the warmest for the closing
- Overnight between coats. A cooling slab can approach the dew point, and moisture on the surface between coats interrupts the bond. Check the dew point before the next coat as well as before the first
When the rule is not enough
The doubling rule is a working approximation of how cure speeds up and slows down, not a property of any one product. It is good enough to show which way a window moves and roughly how far. Three situations call for more than it gives.
- Far from the sheet temperature. Once the slab is more than 10°C (18°F) from the temperature the sheet quotes, the scaled window is more than twice or less than half the printed one, and the rule becomes a rough guide. That limit is our caution, not a published standard. If the sheet has a table by temperature, use it; if not, the manufacturer can usually give a figure
- Below the sheet's minimum temperature. No scaled window applies. The product is outside the conditions it is rated for, and the answer is to bring the slab up and hold it there, not to wait longer
- A heavy coat, still air or high humidity. A film laid heavier than the rated thickness cures more slowly through its depth, and humid, still conditions slow some chemistries and can leave a blush on the surface that has to be washed off before any recoat. None of these is in the temperature rule
Record it as you go
The time each coat finished, the slab temperature during the wait and the time the next coat started are the three numbers that settle a recoat question months later. Written down at the time, they show whether the window was hit on the slab the floor actually cured on. The quality control walkthrough places that record at the hold point between coats, and its printable checklist has a coat log for it.
If the window was missed
Too soon and too late need different responses, and both are covered in full on their own pages. Wrinkling and alligatoring covers what to do when a coat went on before the one beneath it was ready. Intercoat delamination covers the wash, abrade and recoat repair for a window that has closed, which is also what the sheet's instruction for a missed window usually describes.
Frequently Asked Questions
My data sheet says to recoat within 12 to 24 hours. Is the next morning safe?
Only if the slab is close to the temperature the sheet quotes that window at, and often it is not.
Take an illustration sheet that gives 12 to 24 hours at 77°F, and a crew that comes back 16 hours after the last coat. At 77°F that lands inside the window. On a 59°F slab the window opens at 24 hr, so the next morning is too soon. On a 95°F slab it closes at 12 hr, so the same morning is too late.
The overnight gap feels safe because it sits in the middle of the printed window. Whether it is depends entirely on the slab.
How do I scale a recoat window for temperature?
Multiply both edges by the same factor. The working rule is that cure speed roughly doubles for every 10°C (18°F) of warmth and halves for every 10°C of cold, so a slab 10°C colder than the sheet's temperature doubles both edges and a slab 10°C warmer halves them. Between those, the factor moves smoothly. The recoat window calculator on this site does the arithmetic from the figures on your sheet. Where the sheet itself gives a window for your temperature, use the sheet.
What if the data sheet does not say what temperature the window is quoted at?
Look for a footnote under the timing table first, because that is where most sheets put it, often alongside a humidity. If it is genuinely missing, ask the manufacturer before you rely on the window, because a window with no temperature cannot be moved for your slab and cannot be checked afterwards either. Do not assume a temperature and scale from it.
Can I heat the floor to open the window sooner?
You can warm the space and the slab, and on a cold job that is often the right call, as long as it is done before and through the wait rather than in a burst at the end. What you must not do is heat the surface of the coat directly with radiant heaters or a heat gun. A surface warmed much faster than the film beneath it skins over while the body is still moving, which is exactly how a floor wrinkles.
What should I write down between coats?
The time each coat finished, the slab temperature during the wait, and the time the next coat started. Those three numbers are what settle a recoat window question later, because they show whether the window was hit on the slab the floor was actually cured on, not on the printed figure. A quality control checklist with a coat log keeps them in one place.