Recoat Window
The span of time, printed on a technical data sheet, during which the next coat can be applied over the last one and bond to it without any surface preparation. It has an opening and a closing edge, and both move with the temperature of the slab.
What the recoat window is
Most floor systems go down in more than one coat, and each coat has to bond to the one beneath it. The recoat window is the span of time during which that bond happens on its own: the layer below has firmed up enough to carry the next coat, but has not yet cured so far that the next coat cannot knit into it.
A technical data sheet prints it as two figures, a minimum and a maximum time after the previous coat, both quoted at a stated temperature. Inside those two figures the next coat can go straight on. Outside them it either should not go on yet, or needs the surface prepared first.
The opening edge
The minimum is the earliest the next coat can go on. Before it, the layer underneath is still soft enough to be disturbed by what lands on it, and the fresh coat can swell or lift it. The result is a surface that puckers, crinkles or never hardens evenly, which wrinkling and alligatoring covers in detail.
The closing edge
The maximum is the latest the next coat can go on and still bond chemically. After it, the layer underneath has cured into a hard, closed surface that the next coat can only sit on top of. Nothing looks wrong at first, which is what makes this edge expensive: the failure is intercoat delamination, and it tends to appear weeks later, when the top coat peels away cleanly from the one below.
A closed window does not end the job. Most sheets say how to prepare a fully cured surface so it will accept another coat, usually by abrading it. That instruction belongs to the product, so it is the sheet that should be followed rather than a general rule.
Why the window moves
Both edges are set by how far the reaction has gone, and the reaction runs at the pace the temperature allows. A cold slab slows it, so the window opens later and stays open longer. A warm slab speeds it up, so both edges arrive sooner and the window can close before a crew expects it to. The figures on the sheet describe the reference temperature only.
That is why the temperature that matters is the slab rather than the air, and why the same product can behave very differently from one day to the next. Our guide to epoxy recoat windows works through reading the sheet and planning the gap between coats, and the recoat window calculator scales the sheet figures to the slab in front of you.
What it is not
It is not the same thing as tack free. A floor that has stopped feeling sticky tells you about its surface, not about whether it will bond to the next coat, and depending on the product the window may open before tack free or well after it.
Nor is an open window a guarantee the surface is ready. A coat that has picked up dust, moisture or a film of amine blush while it waited can be inside its window and still refuse the next coat. The window answers one question, timing, and the surface has to be clean and dry as well.
Frequently Asked Questions
What happens if I recoat before the window opens?
The coat underneath is still too soft to carry the next one. Depending on the chemistry, the fresh layer can attack or swell the layer below it, which shows up as wrinkling, lifting or a film that never hardens properly.
Our page on wrinkling and alligatoring covers that edge and the causes that sit behind it.
What happens if I recoat after the window closes?
The coat underneath has cured far enough that the next one can no longer bond to it chemically. The new layer sits on top rather than knitting in, and it can look perfect for weeks before it peels away in sheets.
That is intercoat delamination. A closed window is not the end of the job: most sheets describe how to prepare the cured surface, usually by abrading it, so the next coat has something to grip. Follow the sheet for the product you are using.
Why does the window on the sheet not match what happened on my floor?
Because the figures are quoted at one reference temperature, and your slab was probably not at it. A cold slab slows the reaction, so the window opens and closes later. A warm slab speeds it up, so both edges arrive sooner.
The recoat window calculator scales the sheet figures to the temperature of the slab you are actually working on.
Is the recoat window the same as tack free?
No. Tack free only says the surface has stopped being sticky. The recoat window is about whether one layer will bond to the next, which is a different question with its own figures on the sheet.
On some products the window opens before the floor is tack free and on others well after. Read the recoat figures as their own numbers rather than judging them by touch.
Does every product have a closing edge?
No. Some sheets give a minimum recoat time and no maximum, and some give a maximum only under certain conditions. Where a sheet gives no closing edge, read the surrounding notes carefully rather than assuming the window stays open forever, and check the surface is clean before the next coat goes down.