G/flex epoxies

WEST SYSTEM Launches G/Flex Epoxy

By Tom Pawlak

G/flex Epoxy is a toughened, resilient two-part epoxy engineered for a superior grip to metals, plastics, glass, masonry, fiberglass, and wet and difficult-to-bond woods. Introduced in June 2007, G/flex Epoxy is currently available in two consistencies: G/flex 650 Epoxy, a liquid epoxy, and G/flex 655 Epoxy Adhesive, a pre-thickened epoxy. Both have a 1:1 mix ratio.

G/flex Epoxy gives you 46 minute pot life and a long open or working time of 75 minutes at room temperature. It will reach an initial cure in 3–4 hours and a workable cure in 7–10 hours. Wait 24 hours before subjecting joints to high loads.

However, as a marine-grade glue that can be accurately mixed in small batches, G/flex Epoxy offers important benefits.Even as we introduce the new G/flex, we want to emphasize that our existing WEST SYSTEM 105 Resin-based epoxies can’t be beat for versatility and reliability. WEST SYSTEM 105 Resin mixed with the appropriate hardeners and different additives allows you to create an epoxy for a variety of applications like gluing, sealing, fairing, and sheathing. No other epoxy is more reliable for such a huge spectrum of end uses.


Toughness and flexibility
G/flex has been toughened. This gives G/flex the ability to make structural bonds that can absorb the stresses of expansion, contraction, shock, and vibration. G/flex is resilient and impact resistant. With a modulus of elasticity of 150,000 psi (WEST SYSTEM 105 Resin/205 Hardener has a modulus of elasticity of 450,000 psi), G/flex is more flexible and can deflect further before breaking than WEST SYSTEM 105/205, while being much stiffer than typical adhesive sealants.

Adhesion to wet and damp surfaces
G/flex has the ability to glue damp woods. It can be used on wet surfaces, even underwater when applied with specific techniques.

Excellent adhesion to hard-to-bond woods
G/flex adheres tenaciously to difficult- to-glue hardwoods, both tropical and domestic varieties. This is important since many of the exotic and tropical species now being used to replace traditional woods present bonding challenges.

Bonds well to a variety of materials
G/flex® is ideal for bonding a variety of materials, including dissimilar ones. G/flex has a superior grip so that it can be used to bond to metals, plastics, glass, masonry, and fiberglass. G/flex is ideal for repairs to aluminum boats and polyethylene and ABS canoes and kayaks. It can also be used to wet out and bond fiberglass tapes and fabrics.

Ease of use
G/flex is a simple two-part epoxy system. Resin and hardener are mixed in a 1-to-1 mix ratio by volume. G/flex provides a relatively long open working time, yet it cures quickly and can be used in cool temperatures. Because it is simple to mix and use, G/flex is an excellent starting point for customers new to epoxy use.

G/flex can be modified with WEST SYSTEM® fillers and additives if you need to meet particular bonding needs. Adding G/flex to other WEST SYSTEM epoxies can improve their toughness and flexibility.


G/flex is the result of years of experimentation to develop a formula for a toughened epoxy. We wanted something that was simple to use, viscous enough not to drain out of a joint, and adhered tenaciously to a variety of materials under difficult conditions. As explained more fully later (see Understanding Flexible Properties, p. 17), material properties of an epoxy form a complicated web. When you formulate for specific end properties (like high elongation), you usually have to give up other properties in order to achieve it. Some of us thought that if you formulate an epoxy with five to six times the tensile elongation of other WEST SYSTEM epoxies, the new product would be poor at dealing with constant or long duration loads. Yet when we tested G/flex under long duration loading with our exclusive Creep Test, it performed admirably—nearly matching the 105 Resin-based epoxy.

We also wondered if an epoxy with this much elongation would perform poorly in heat resistance. Yet ASTM-D648 (Heat Deflection Under Load or HDUL) revealed G/flex performed even better than 105 Resin-based epoxies. (G/flex like all WEST SYSTEM resin/hardener combinations can handle temperatures up to 200°F repeatedly. At this temperature, it will be more flexible and less resistant to heavy durational loads than at room temperature, but it returns to full strength as it approaches room temperature.) Well, what about through cure? Usually flexible systems take days to achieve the majority of their physical properties. Our testing revealed that G/flex’s 24-hour through cure is similar to that of WEST SYSTEM 105 Resin and 205 Fast Hardener.


How does G/flex adhere to woods and metals? Adhesion testing using the PATTI (Pneumatic Adhesion Tensile Test Instrument) on the same pieces of wood (with the wood sanded. Parallel to the grain with 80-grit sandpaper but no solvent wash) revealed that G/flex adheres to wood at least as well as any other WEST SYSTEM epoxy. With hardwoods and the often difficult-toglue species such as white oak, Ipe, teak, greenheart, purpleheart and salangan batu, G/flex performed as much as 30% higher.

Adhesion testing with G/flex Epoxy on metals also yielded excellent results, typically exceeding the adhesion results achieved with 105 Resin-based epoxies (see Figure 1).

Surface Prep/Conditions Tensile Adhesion (psi)
G-10 high-density laminate 650 80-grit sand / dry surface 3459
G-10 high-density laminate 655 80-grit sand / wet surface 2473
1018 steel 650 80-grit sand / dry surface 1772
1018 steel 655 80-grit sand / wet surface 3562
Galvanized steel 650 100-grit wet sand 2562
Galvanized steel 655 100-grit wet sand 2929
Copper 650 80-grit sand 2334
Copper 655 80-grit sand 2685
Bronze 650 80-grit sand 2782
Bronze 650 Scotch brite™ pad sand 2962
Bronze 655 80-grit sand 2936
HDPE plastic 655 Alcohol wipe, flame treat 1885
ABS plastic 655 80-grit sand 1535
Lexan™ 655 80-grit sand 1870
Ipe 650 60-grit sand 2134
Ipe 655 Plane, isopropyl alcohol wipe x 3 2223
Teak, vertical grain 650 80-grit sand parallel to grain 1413
Teak, vertical grain 655 80-grit sand parallel to grain 1381
Teak, vertical grain 655 80-grit sand, alcohol wipe x 2 1503
White oak, vertical grain 650 80-grit sand 1935
White oak, vertical grain 655 80-grit sand 1780
White oak, vertical grain 655 Alcohol wipe x 2 2212
Purpleheart 650 60-grit sand parallel to grain 1731

We also found that G/flex® 655 Thickened Epoxy Adhesive adheres to wet and damp surfaces well. Obviously gluing to wet surfaces, especially when dealing with absorbent substrates like wood, is less than ideal because water is taking up the spaces where epoxy otherwise would find its way in; however, G/flex worked surprisingly well. Technique plays an important role in how effective a wet surface adhesion (even underwater repairs) will be. The epoxy must be thick enough to displace the water to ensure a good bond. Pre-thickened G/flex 655 Adhesive (or G/flex 650 that has been thickened with 406 Colloidal Silica to a mayonnaise consistency) is needed.

G/flex Kit instructions include a number of short “how-to’s” on gluing to damp and wet surfaces, performing underwater repairs, and repairing plastic boats, including crack repairs and making skid plates for worn ends on plastic canoes.


Adding G/flex to WEST SYSTEM 105 Resin-based epoxy improves toughness and tensile elongation. Using it this way expands the utility and versatility of the WEST SYSTEM product line even further. G/flex can also be used with WEST SYSTEM G/5 Five-Minute Adhesive to extend the working time. The more G/flex added to G/5, the slower the cure and the tougher the cure properties become.


With all the attributes and improved properties of G/flex, you might be asking whether you still need WEST SYSTEM 105 Resin-based epoxy? The answer is that G/flex can’t do some things as well as 105 epoxy. Examples include barrier coating and fiberglassing with heavier fabrics. Although G/flex flows nicely when spread out on a surface, it is less than ideal as a coating because of its higher viscosity. WEST SYSTEM 105 epoxy is better for wetting out fiberglass cloth, especially for clear finish projects like wood strip canoes and kayaks. WEST SYSTEM 105 epoxy is also a better base for creating fairing putties because its lower viscosity allows you to add more low-density filler to it. This translates into a fairing putty that sands and carves more easily because of the higher filler loading.

Several articles follow that will help you further understand our new G/flex toughened epoxy and its properties. We encourage you to read these and then experiment with G/flex as we are doing. We think you will find many projects for which the particular properties of G/flex are ideally suited. As always, our Technical Staff is available to answer your questions, and we will be eager to hear about your projects and repairs using the new G/flex Epoxy.


This is a good time to clarify that there are now three different epoxy types in the WEST SYSTEM product line. 

1.What we previously referred to as WEST SYSTEM epoxy are the four resin/hardener combinations based on 105 Resin(105 Resin mixed with 205 Fast Hardener206 Slow Hardener207 Special Clear Hardener or 209 Extra Slow Hardener).

2. G/5 Five-Minute Adhesive is a single epoxy resin/hardener combination.

3.We have now added G/flex Epoxy to the WEST SYSTEM product line. G/flex includes 650 (liquid) Epoxy and 655 (thickened) Epoxy.

All of our WEST SYSTEM epoxies are cure-compatible. If you wish to blend one with another, first mix on ratio the resin and hardener from one of our epoxies, then add it to mixed on-ratio resin and hardener from another. For example, to speed the cure of 105/205 you may mix a batch of this at the correct (5:1 – one pump each with our calibrated mini pumps) then blend with mixed 1:1 ratio G/5.