Should I use a 5:1 or a 2:1 epoxy system?

All epoxies are not created equally, but they all have basic properties you'll want to consider before picking the one that's suitable for your project.

Mix ratio (e.g., 5:1 or 2:1) should not necessarily dictate which epoxy system to use. Instead, your choice should be centered around either the working properties or the cured properties, such as working time, tensile strength, hardness, clarity, HDT (heat deflection temperature), blushing, casting, modulus, flexibility, etc.

Mix Ratio - Epoxy systems have two components: resin and hardener. Ratios vary by epoxy, but depending on the type of resin, they can be 1:1, 2:1, 3:1, or 5:1. The first number is the amount of resin to measure (by volume); the second number is the amount of hardener to measure (by volume). EXAMPLE: A 2:1 mix ratio means that you need to measure 2 parts resin to 1 part hardener, by volume. So 4 oz. of resin would require 2 oz. of hardener. Epoxy resins are designed to work with specific hardeners at precise ratios. If you try to add more hardener to make the epoxy cure faster, you'll end up with a gooey mess that doesn't cure.

Pot Life - Refers to the amount of time it takes for the mixed liquid resin to harden enough (at a specified temperature) that it doubles in viscosity. EXAMPLE: If the pot life is 10 minutes, the resin will be twice as thick 10 minutes from the time the resin and hardener components were mixed.

Working Time - Also known as working life, this is the amount of time (at a specified temperature) the mixed epoxy remains in a viable, liquid state so it's still workable. EXAMPLES: BONDING - If the working time is 20 minutes, you have 20 minutes to apply clamping pressure to your assembly. After that, you can no longer assume you will get a dependable bond. CASTING: If the working time is 30 minutes, you have 30 minutes to pour and add additional epoxy-safe color pigments before the resin will be too thick.

Cure Time - Refers to the time it takes for the mixed epoxy to change from a liquid to a solid. Cure time varies based on the type of hardener used (different hardeners slow down or speed up the cure rate); the working temperature (epoxy cures faster in warmer conditions and slower in cooler conditions); and the quantity of mixed epoxy (large quantities of mixed epoxy generate more heat, resulting in a faster cure, and the opposite is true with smaller quantities of mixed epoxy). 

Time to De-mold - Refers to the window of time allowed for safe removal of the cured epoxy from the prepared mold.

Time to Sand - This is the amount of time you must wait, at a given temperature, before the cured epoxy is hard enough to sand.

Maximum Single-Layer Thickness per Pour - This property applies to coating, casting, and encapsulating epoxies, and refers to the maximum thickness you can pour a single layer, either onto a prepared surface or into a prepared mold. Typically, a casting epoxy can be poured in a thicker single layer than a coating epoxy.

UV Protection - If the cured epoxy is part of the surface finish, and it will have continuous UV exposure, be sure to find out whether the epoxy has any UV inhibitors, because many do not. If the finished project needs UV protection, you need to either use an epoxy with UV blockers, or coat the finished piece with a UV-filtering varnish or clear coat polyurethane.

Hardness - Each epoxy has a different hardness level, measured as Shore D Hardnness. The higher the number, the harder the cured resin will be.

Strength - If strength is a concern, check the flexural strength and tensile strength properties of the epoxy system.

Flexibility - Some cured epoxies are more flexible than others. EXAMPLE: A 2:1 epoxy system will have greater flexibility post-cure than a 5:1 epoxy system.

Clarity - The clarity of epoxy systems runs the gamut from crystal clear to light amber. If water clarity is important to your project, check the cured color of any resin systems you're considering.

HDT - Heat Deflection Temperature is the temperature at which a cured polymer or plastic deforms under a specified load.

Understanding these epoxy properties will help with epoxy selection for your specific application. The table below lists epoxies that are good for specific types of projects. Be sure to do your research before making your final decision.