Epoxy vs. Resorcinol

Problems, how to use. Also see: "EPOXY", in the left-hand column of the Home page.

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Locutus
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Epoxy vs. Resorcinol

Postby Locutus » Sat Mar 07, 2015 11:52 pm

Much ado is made about the superior properties of Epoxy products, and no wonder, considering how expensive they are, and how profitable for the companies selling it. But is epoxy really the best adhesive for boatbuilding, if so, then why, for example, does marine plywood use Resorcinol and not epoxy as its adhesive?

Larry Pardey, in his book Classic Boat Construction, Appendix B, discusses and compares various adhesives used in boat construction. Among others, Epoxy and Resorcinol are given special attention. The following paraphrases what I learned from this appendix.

Epoxy is very expensive for the buyer, and very profitable for the seller. Resorcinol is relatively cheap and has been around for many decades. You may wonder why epoxy is promoted so much, while Resorcinol is hardly ever mentioned. For one thing, boatbuilding represents a tiny fraction of the market for Resorcinol, the bulk being the construction industry. The manufacturers of Resorcinol have little incentive to promote it to wooden boat builders.

On the other hand, the manufacturers of epoxy products have a vested interest in the boatbuilding industry, both for wooden and fiberglass construction. And as already mentioned, it's priced to generate large profits for the manufacturers. So suppliers to boat builders have a DIS-incentive to sell Resorcinol. In fact, just try to buy Resorcinol at your local marine supply. Chances are they not only won't stock it, but they probably can't even special order it. You may have to search online for it yourself.

So besides the lower cost, you might be wondering why you would want to seek out Resorcinol instead of the more available but higher priced epoxy. After all, epoxy is easier to work with because of its gap-filling ability. You don't need perfect joints or strong clamping pressure as you might with Resorcinol. Although, that last point may prove to be a misconception, as we'll see in a moment.

Larry Pardey not only built two of his own yachts (Seraffyn and Taleisin) and restored a 100 year old classic sailboat for himself and his wife Lin, but has for decades made a living out of repairing other people's boats. In his Appendix B he gives several examples of epoxy joint failures, including one boat about ten years old literally coming apart at the seams. This boat's planking and structure was bonded throughout with epoxy.

In the thirty years leading up to authoring Classic Boat Construction, Larry Pardey had seen many examples of epoxied joint failures, but not one Resorcinol joint failure. Even on his own boat Taleisin, in which the majority of the structure used Resorcinol, but some of the cabin top structure used epoxy, the few joint failures were all epoxy failures, even in cases where both epoxy and Resorcinol were used in the same structure in close proximity.

Some other considerations:

Epoxy is not actually waterproof. It's only rated water resistant. Over time (we're talking years), water can slowly penetrate through epoxy sheathing and raise the moisture content of the underlying wood, little by little.

Epoxy is temperature sensitive. This is especially important if your boat is headed for the tropics. A white painted boat deck can reach temperatures in excess of 160* F on a sunny day in low latitudes. Most epoxies start losing strength at about 120* F. Resorcinol on the other hand, is rated WBP (Water and Boil Proof). You can throw a piece of marine plywood laminated with Resorcinol into a pan of boiling water and it will not delaminate.

There are actually two kinds of Resorcinol. The first is the one you may be already familiar with, that requires strong clamping pressure and a perfect joint (max gap 1/32"). The other is more forgiving. It can be applied similarly to epoxy and joints can have gaps up to 1/16". Larry Pardey refers to the latter as "Winter Grade Resorcinol." He gives a manufacturer name in Britain. I don't remember the name, sorry. I don't know if this Winter grade is available from a US supplier.

A new epoxy bond is, in fact, stronger than the surrounding wood. However, its useful lifespan is shorter than the wood. Larry Pardey mentions 20 years as a rule of thumb, in ideal conditions. Part of the reason for this is the fatigue that occurs over time with the constant flexing that happens while underway, with the boat's motion in a seaway. Wood can flex millions of times without losing any of its strength. (Wood is the only material with this property.)

The companies that sell epoxy for marine use don't actually manufacture it. They buy the basic "building blocks" from the few large chemical companies and using these basic materials, combine them to make their own proprietary formulations. Then put their own label on them and jack up the prices.

According to Larry Pardey, epoxy does have it's place, but he would reserve its use for fiberglass sheathing and encapsulation, not for structural joints, with due regard for the fact that it's only water resistant. An additional exterior water barrier coat below the water line may be warranted.

Have any of you used Resorcinol for your Glen-L boat build? If so, what was your experience with it?

I'm also interested in what Glen-L's opinion is on this. Gayle or Glen, care to comment?

Mark Shank
Last edited by Locutus on Fri Jun 19, 2015 10:32 am, edited 2 times in total.

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jamundsen
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Re: Epoxy vs. Rescorcinol

Postby jamundsen » Sun Mar 08, 2015 10:17 am

April 24, 2012 699.JPG
I used Resorcinol once while building a wooden airplane. It was difficult to use compared to epoxy. Joints had to be perfect to prevent structural failure with very high clamping pressures. Once the FAA decided epoxys could be used on aircraft for building and repairs I switched. The Gougeon Brothers have been building high stress boats(racing trimarans) for 50 years with epoxy and wood. They might be a better source of information on longevity since most of their boats are still operating.
I did all the repairs on this Boeing Stearman with Epoxy.
John Amundsen
Monte Carlo
Lakeland,Fl

Work tends to get in the way of boat building

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Bill Edmundson
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Re: Epoxy vs. Rescorcinol

Postby Bill Edmundson » Sun Mar 08, 2015 10:26 am

I am not the world's best woodworker. My joints are rarely tight. Epoxy can fill the gaps. Most of us will only build a few boats. People tend to think their boats can go anywhere or find themselves in a bad situation. I would rather go with the tried and true. I use a good bit of construction adhesive in none critical areas. My test have shown me the wood failing before the glue.

Bill
Mini -Tug, KH Tahoe 19 & Bartender 24 - There can be no miracle recoveries without first screwing up.
Tahoe 19 Build

slug
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Re: Epoxy vs. Rescorcinol

Postby slug » Sun Mar 08, 2015 5:27 pm

IMHO,I.ve always had the impression that Larry Pardey was an expert, and if you're uncertain, just ask him. It's been referred to as "The Pardey Line".

Doug


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