How to read an epoxy technical data sheet

A technical data sheet (TDS) is the manufacturer’s own summary of how a product behaves. It is also the only place most numbers on this site come from. This guide explains the common fields, what they measure, and the fine print that decides whether a number applies to your project.

Rule one: every number has conditions

Almost every value on a TDS was measured under stated conditions: a temperature, a mass of mixed resin, or a thickness. Change the conditions and the number changes. Gougeon Brothers, for example, notes that the pot life on its WEST SYSTEM data sheets refers to a confined 100 g mass at 72 °F (Epoxyworks).

That is why product pages on this site show conditions next to each value, and why our calculators do not stretch a published time to a different temperature.

Mix ratio: by volume and by weight

Most data sheets give the ratio of Part A to Part B one way or both ways, for example “2:1 by volume” and “100:44 by weight”. The two are not interchangeable: resin and hardener usually have different densities, so the weight ratio usually differs from the volume ratio (TotalBoat).

Some products are meant to be measured one way only. Smooth-On’s EpoxAcast bulletin, for instance, publishes a ratio by weight and no volume ratio (see EpoxAcast 692 Deep Pour). When the data sheet also gives the specific gravity of each part, the mix ratio converter can translate between the two and labels the result as computed.

Pot life and working time

Pot life (sometimes gel time) is how long a mixed mass stays liquid in its container. Because it is measured on a confined mass, heat builds up and it runs short.
Working time is how long you can work the resin once it is spread or poured.

Epoxyworks explains that working time always exceeds pot life, since epoxy confined in a pot reacts faster than epoxy spread in a thin film, and that spreading the epoxy out extends working time (Epoxyworks). Many deep-pour data sheets publish only a working time in hours; many coating data sheets publish both.

Time between pours, recoat window, tack-free

These fields tell you when the next layer can go on:

Minimum time between pours is the earliest the next layer should be poured, often with a condition such as “firm” or “below a set temperature”.
Recoat window is the period in which the next coat bonds without sanding. After it closes, expect to sand first; Promise Table Top, for example, asks for light sanding once more than 10 hours have passed.
Tack-free is when the surface no longer feels sticky. It is not the same as cured.

Some data sheets describe the window in words (“when tacky but firm”) rather than hours. Our database records that wording and leaves the number blank rather than inventing one.

Demold and full cure

Demold is when a casting can come out of its mold. Full cure is when the resin reaches its final properties, and it is typically measured in days. Handling or loading a piece between the two can mark or distort it.

Maximum pour depth

The most important number for deep pours. It is usually stated per pour or per layer, and sometimes depends on batch size or mold type. Treat it as a ceiling at the stated temperature, not a target. The layer planner uses it directly.

Viscosity

Viscosity measures resistance to flow, usually in centipoise (cP or cps). Smooth-On’s reference scale puts water at 1 cP and honey at 2,000 to 10,000 cP, and notes that for resins, warmer means lower viscosity (Smooth-On). On a TDS, look for whether the figure is for Part A, Part B or the mix, and at what temperature. Low viscosity helps a deep pour flow into gaps and release air; higher viscosity helps a coating stay on edges.

Hardness (Shore D)

Cured epoxy hardness is measured with a durometer on the Shore D scale. Epoxyworks describes the test (ASTM D2240) as forcing a metal point into a slab of cured epoxy and reading its resistance, and notes that hardness increases with degree of cure, so the cure time at testing matters (Epoxyworks). Shore scales overlap; Smooth-On gives the example that Shore 95A is also about Shore 45D (Smooth-On).

Heat deflection temperature (HDT)

HDT tells you how warm a cured piece can get before it starts to deform under load. Epoxyworks describes the test (ASTM D648): a bar of epoxy under a constant 264 psi load is heated in oil until it deflects 0.1 in, and that oil temperature is the HDT (Epoxyworks). It is a lab benchmark, not a promise that a hot pan is safe.

Specific gravity and density

Specific gravity is density relative to water. On a TDS or SDS it may be given per part or for the mix, sometimes as a weight per gallon instead. It is what lets you convert a volume ratio to a weight ratio. SDS section 9 (“physical and chemical properties”) is often where per-part values appear.

What is not on the TDS

Hazards and protective equipment are on the safety data sheet (SDS), not the TDS. Read the SDS for both parts.
Prices, kit sizes and shelf life may be on the label or product page instead.
Your conditions. The TDS describes a test, not your shop. When your conditions differ, look for the manufacturer’s own guidance on adjusting, and ask them if there is none.