Deep pour vs table top epoxy
Two clear liquids that look identical in the bottle, with maximum pour depths about ten times apart. Going past the wrong one is a heat problem, not a patience problem.
Table top epoxy pours to about 1/8 in in one coat and deep pour epoxy to about 2 in in one lift — roughly a tenfold difference. Exceeding it is not a slow cure: epoxy gives off heat as it cures, that heat speeds the reaction up, and a deep mass of a fast resin runs away with itself, cracking the piece, yellowing it, smoking and scorching what it stands on. To build real thickness, use a casting resin, or pour a coating epoxy in repeated thin coats and let each one firm up first.
The two bottles look the same. Both say epoxy, both are clear, both mix a resin with a hardener, and one of them will do something genuinely dangerous if you use it for the other's job. Table top / coating epoxy is formulated to be poured about 1/8 in at a time; deep pour / casting epoxy goes to about 2 in in a single lift. That is not a quality difference or a marketing one. It is the whole design.
Epoxy cures by a reaction that gives off heat, and that heat makes the reaction go faster, which makes more heat. In a thin film the warmth escapes into the room and nothing happens. In a deep mass it cannot, so the pour climbs to a temperature it was never meant to reach — the piece cracks and yellows, the resin smokes, the mold distorts, and a large enough mass of the wrong resin can scorch the surface it is standing on. This is why a coating epoxy and a casting epoxy are different products, and it is the only figure on this site where being over is a hazard rather than a disappointment.
The rest of this page is the practical version of that: what each system's ceiling actually is, what you can do when your project is deeper than it, and how long to wait between lifts. If you already know your depth, the calculator will check it against whichever system you have picked and tell you how many pours it needs.
What each depth needs, in pours
Both columns come from the same function the calculator's warning uses, so this page and that tool cannot state different ceilings. Table top is 1/8 in per coat; deep pour is 2 in, and some systems state up to 4 in in a cool room. Your own resin's technical data sheet overrides both, in either direction.
The two ceilings, and why they are different
Per flood coat. It reaches peak temperature within about 20–40 min of mixing, which is exactly what makes it convenient and exactly what makes depth dangerous — the heat has to escape into the room in that window, and in a thick pour it cannot. Mixed 1:1, handled in 24 hours, hard in 3 days.
Per lift, with some systems stating 4 in in a cool room, a wide mold and a thin mass. It is chemically slowed down so that 3–5 hours of working time and 7 days to full cure buy it the ability to shed its own heat. Mixed 2:1 by volume, which is 2.20:1 on a scale.
Depth is not the only way to trap heat. A tall narrow cup holding more than about 0.9 litres of a coating epoxy can start to smoke while you are still stirring it, because the cup is a deep pour in its own right. Mix large amounts in several batches and in a wide vessel, and get each one out of the cup and onto the work before starting the next.
What actually happens when you go past it
Curing epoxy releases heat. Warmer epoxy cures faster. Faster curing releases heat faster. In a thin film that loop is broken by the room, which carries the warmth away as fast as it appears — the piece barely gets above room temperature. In a deep mass the middle has nowhere to send it, so the loop closes and the temperature climbs on its own. The technical term is a runaway exotherm and the useful word is "sudden".
The surface goes soft and rubbery rather than hard, then wavy. The mass turns amber and then brown. Cracks open through the middle, often with a sound. It smokes, and the smoke is acrid and genuinely worth leaving the room for. A large enough mass gets hot enough to scorch a table, deform a silicone mold permanently and, at the far end, ignite what it is sitting on. None of this is recoverable and none of it is slow once it starts.
A pour at the ceiling in a 15 °C garage behaves differently from the same pour in a 28 °C summer kitchen. Narrow and tall is worse than wide and shallow at the same volume, because the middle is further from any surface. Silicone insulates, so a mold traps more heat than an open form of the same shape does.
The mass in the mixing container is subject to exactly the same physics as the mass in the mold, and it has less surface to lose heat through. This catches people out on flood coats: the pour itself is 1/8 in and perfectly safe, but the 2839 ml sitting in the bucket beforehand is not.
A casting epoxy poured thin is fine — slow, but fine. A coating epoxy poured thick is not, and no amount of care changes that. If the project is deep and the coating epoxy is what you own, the answer is 4 coats rather than one thick pour, or buying the right resin.
Building real depth safely
Divide the finished depth by your resin's stated maximum and round up. River table, 6 ft at 45 mm is 1 pour of about 18.1 litres in a casting epoxy. The calculator does it from your own dimensions.
A lift that has gelled but not fully cured bonds chemically to the next one and the join disappears. A fully cured lift bonds mechanically only, so it needs a light sand with 220 grit first, and even then the layer can show as a faint line in raking light. The window is usually a few hours for a coating epoxy and overnight for a casting one — your resin's data sheet calls it the recoat window.
The usual advice to warm resin is about viscosity and bubbles, and it applies to the bottles before mixing, not to a deep pour afterwards. For a lift at or near the ceiling, a cooler room is straightforwardly safer — it widens the margin. Do not put a deep pour anywhere near a heater to speed it up.
If you have a choice about the form, give the mass more surface. The same volume in a wide shallow mold runs cooler than in a tall narrow one, and a deep narrow cavity is where an exotherm shows up first even when the stated depth is fine.
Mix no more than about 0.9 litres of a fast resin at once, in a wide container rather than a tall one, and pour it out as soon as it is combined. Resin left standing in a cup is heating up; resin spread over a table is not.
There is nothing to salvage at that point and the fumes are the immediate problem. Move it outdoors if you safely can, ventilate the room, and let it finish where it cannot set fire to anything. Do not put water on it and do not try to pour it out.
Check your depth before you mix
Enter your project and its depth, pick the resin you actually own, and the calculator gives the volume, the resin and hardener split and how many separate pours the depth needs — with the exotherm warning if it is past what that system can take.
Open the epoxy calculator →Print the pour-depth card
Maximum pour depth and cure windows by resin type — the number that cracks a casting. Free.
Estimates, not advice. Every figure on this page is computed from published reference values and standard measures, so your own inputs, equipment and conditions will move it. Nothing here is professional advice — talk to a qualified professional before acting on it.