Penetrating epoxy: legend or myth?

I’m rapidly approaching my 22nd year of employment at Gougeon Brothers, Inc. Since my first day on the job, I’ve been immersed in the debate between penetrating epoxies and high-solids epoxies in general, and WEST SYSTEM® Epoxy in particular. I’ve had numerous conversations with the formulator and founder of a popular brand of penetrating epoxy, engaging in discussions over the years. He’s a pleasant individual and deeply passionate about his product, but ultimately, we had to respectfully agree to disagree.

A brief online search provides an overwhelming amount of information, arguments, and sometimes less-than-civil discourse on the topic. I don’t claim to always be the most rational person, so let’s dive right into it.

We aren’t particularly enamoured with penetrating epoxies. In fact, we don’t entirely see the necessity for them. At least not as some folks seem to perceive it: “I just want the most robust bond possible to the wood,” or one of my personal favourites, “I need to consolidate the dry rot.” Of course, because using weak, porous epoxy on deteriorated wood magically transforms it into something incredibly sturdy—just like new!

I apologise if I sound a tad sarcastic. I couldn’t resist. But I do have valid reasons for my playful rebuke. One aspect I can’t deny is that the leading manufacturers of penetrating epoxies are masters of marketing. I won’t divulge specific names, but it’s not difficult to comprehend why someone might believe statements like, “Our Penetrating Epoxy is derived from wood products, making it more compatible with wood,” or “Our penetrating epoxy is a ‘high solids’ epoxy.” As the legendary P.T. Barnum once wisely quipped, “There’s a sucker born every minute.”

A brief perusal of the SDS (Safety Data Sheet) for a penetrating epoxy holds all the essential information you’d need—provided you’re a chemist. Fortunately, I happen to possess a chemistry background, and here’s what I uncovered.
A look at Penetrating EPOXY A

One of the top-selling penetrating epoxies, referred to here as EPOXY A, is composed of a substantial 69% solvents. This signifies that merely 31% of this product consists of epoxy resin and amine hardener. The resin and hardener of EPOXY A are sourced from standard chemical providers like NANYA, Olin, or various manufacturers across the globe. However, EPOXY A omits any mention of typical epoxy components such as Bis-A resin, polyamines, or nonyl phenol. Their listing comprises solely the minimum hazardous solvent ingredients, as mandated by government regulations. The Volatile Organic Content (VOC) stands at 675 g/ltr, surpassing the legal VOC limits in all parts of the US. It remains legally permissible to distribute this product due to the primary entry on the hazardous list, enigmatically termed “aromatic hydrocarbon 64742-95-6.” We possess the capability to recognize compounds like this through their CAS (Chemical Abstract Service) number. As it turns out, this cryptic “aromatic hydrocarbon” is, in fact, #1 Naptha, commonly recognized as white gas or Coleman® fuel. #1 Naptha is classified as an exempted solvent. EPOXY A incorporates an adequate amount of it in its formulation to conform to the legal selling requirements in the U.S.

Aromatic napthas are derived from coal and coal tars, which in turn originate from wood. Therefore, there is a modicum of misleading truth in the assertion that their penetrating epoxy is created from “natural and earth-friendly” origins. Nonetheless, the issue arises from the fact that the “natural” or “wood derived” aspect of EPOXY A primarily stems from solvents such as Naptha and isopropyl alcohol, rather than the implied epoxy component.

Naptha has the capacity to penetrate wood cells with ease. Regrettably, this solvent also segregates from the epoxy, leading to a situation where the epoxy doesn’t permeate the wood along with the solvent front.
A look at Penetrating EPOXY B

Now let’s look at EPOXY B, a newer ‘high-solids’ penetrating epoxy on the market. The claim ‘high-solids epoxy’ implies two things. First, that the product is not solvented and second, that it’s as strong as true high-solids structural adhesive epoxies like WEST SYSTEM and others.

The SDS of EPOXY B is more detailed than that of EPOXY A, but it reveals an epoxy resin and hardener content in only the 40%–60% range. The other ingredients—while not nearly as flammable, toxic, or nasty as the solvents in EPOXY A—are considered non-reactive diluents, meaning they don’t react with the solids in epoxy or become part of the cured epoxy matrix. EPOXY B does not constitute a true high-solids epoxy.

What EPOXY A and EPOXY B have in common is poor moisture exclusion properties. Some of these solvents have long-chain molecules, and since they are non-reactive and/or volatile, they will find their way out of semi-cured and even cured epoxy coatings. This leaves behind ‘wormholes,’ which water, with its small H2O molecule, can easily penetrate.

While I would anticipate EPOXY B to offer somewhat improved moisture exclusion compared to EPOXY A, neither can match the performance of the unsolvented WEST SYSTEM epoxy.

We conducted a test to compare the moisture exclusion capabilities of penetrating epoxy and WEST SYSTEM 105/205 epoxy. The test involved applying 3 coats of 105/205 or high-solvent epoxy (EPOXY A) onto prepared and dried plywood samples. We then measured the weight increase of these samples when exposed to an environment with 27°C/90% relative humidity over a certain period. The graph should be interpreted as follows: The high-solvent epoxy (EPOXY A) sample gained 90% of its weight. This amount is more than four times the moisture absorbed by the 105/205 sample. Clearly, solvented, penetrating epoxies do not serve as effective moisture barriers, with only a 10% effectiveness after six weeks of exposure.
Getting Soaked

What about the epoxy “soaking in” deeper for better adhesion? WEST SYSTEM and other true high-solids, quality epoxies have never relied on the so-called deep penetration to establish an effective bond. Our products offer robust and effective bonding, along with moisture-resistant coatings that maintain the wood’s moisture content at a stable level. Dry wood is sturdy wood. Our philosophy is founded on the idea that wood serves as an exceptional structural engineering fibre when kept dry. Our 47-year history stands as evidence of this principle. A notable illustration is the case of Meade Gougeon’s 35-foot trimaran, Adagio, launched in 1970. Adagio arguably stands as the oldest wood/epoxy structure worldwide, and not a drop of penetrating epoxy was utilised in her construction or maintenance. The essential point to remember is this—achieving an effective moisture barrier doesn’t necessitate the epoxy to deeply penetrate the wood. WEST SYSTEM Epoxy, along with other fully solid epoxy adhesives, penetrates sufficiently to induce wood failure in both tensile adhesion and shear stress tests.
Maximum Penetration

Over the years, we’ve acquired a few techniques to achieve maximum penetration using standard WEST SYSTEM resin and hardeners. The first method involves using the 209 Extra Slow Hardener. The extended open time of 3-4 hours, compared to the 90–110 minutes of the 206 Slow Hardener, permits the epoxy to permeate more effectively.

Another approach to enhance penetration into wood is to preheat the wood. Elevate its temperature by around 30–35°C above the ambient temperature using a heat lamp or heat gun. As the wood warms up, the air within its cellular structure expands and exits. This phenomenon is referred to as outgassing. Thus, warm the wood and prepare your epoxy mixture. Once the wood reaches the desired temperature, remove the heat source and apply the epoxy onto the cooling wood. Two key events transpire: when the epoxy contacts the warm surface, its viscosity reduces. Subsequently, as the wood cools down, the air within its cells contracts, drawing the thinned, warmed epoxy along with it. It’s worth noting that heating the epoxy will curtail its available working time.
Comparison Test

I conducted a series of comparative tests involving EPOXY A and WEST SYSTEM, where I introduced different solvents to WEST SYSTEM 105/206 epoxy at a concentration of 5% by weight. I utilised clear Douglas fir blocks measuring 3/8″, 5/8″, and 7/8″ in thickness. These blocks were obtained from the same board, with the grain oriented through the thickness, akin to end grain balsa. I weighed 10g of the mixed product and applied it to the tops of these blocks. While the epoxy cured, the blocks were positioned on pieces of plywood, enabling the epoxy to migrate through the blocks and into the plywood. All of these procedures were conducted at room temperature (approximately 22°C).
Above, we observe EPOXY A and WEST SYSTEM 105/206. Firstly, we notice the glossy coating of 105/206 on the surface, as opposed to the dull appearance of the top of the EPOXY A blocks. The test result table on the right showcases that the undersides of the EPOXY A blocks largely resemble their tops, while the bottoms of the 105/206 blocks exhibit dots of epoxy that managed to penetrate through to the end grain. This visual difference might lead one to speculate that EPOXY A penetrated deeper into the wood. But does it?

Despite these appearances, the EPOXY A blocks could be lifted off the plywood with ease—there was no adhesion at all. If any epoxy solids had truly penetrated through, there should have been some degree of bonding with the plywood. However, this did not occur.

On the 105/206 blocks, even though it might seem like less epoxy managed to penetrate through, a sufficient amount did so to cause wood failure (as opposed to failure at the glue line) of the underlying plywood. Detaching these samples required tapping them with a hammer.

These outcomes support my longstanding theory that in so-called penetrating epoxies with a high solvent content, the epoxy component does not traverse the wood alongside the solvents. This theory is further substantiated by the fact that none of the solvented instances of WEST SYSTEM 105/206 experienced plywood base failure, unlike the unaltered WEST SYSTEM epoxy. The solvented samples had to be tapped with a hammer, indicating that some epoxy moved with the solvent front, although insufficient epoxy was present to induce plywood surface failure. It’s worth noting that these results were obtained with only 5% solvent added to WEST SYSTEM 105/206, while EPOXY A contains 70% solvent and EPOXY B is composed of at least 40% diluent and potentially up to 60%.

As is evident, the wet or stained regions seem quite extensive in the WEST SYSTEM samples. However, once again, not enough structural high-solids epoxy managed to penetrate through these blocks to securely adhere them to the plywood.

In conclusion, solely the solvent-free WEST SYSTEM 105/206 demonstrated the strength necessary to cause the plywood base to fail. I pose the question back to those who advocate for the superiority of penetrating epoxies. Cats don’t really wear pyjamas, do they?

By Bruce Niederer — GBI Technical Advisor