Epoxy for Joinery in the Marine Environment

Whether as an adhesive or a finish, epoxy will face unusual stresses when used for yacht construction. January 12, 2009

I am currently outfitting a 62 foot conversion yacht with mahogany, padouk, and port orford cedar. I will be using epoxy joinery and finishing all brightwork with epoxy. Has anyone done a similar project? Any advice on epoxy construction?

Forum Responses
(Adhesive Forum)
From contributor J:
Help me out: what is "epoxy joinery?"

From contributor E:
I wouldn't depend just on any adhesive for yacht joinery. Spent 20+ years in the field doing wooden boats and interiors. Boats face wide ranging humidity and temp conditions and solid wood is always moving in the joinery. I always used adhesive and mechanical fasteners and as far as I know, nothing ever came apart. Epoxy can make a good base coat for clear topcoats but is not a very good finish on its own. Use a good linear urethane over it or plenty of good old marine varnish (like 7 coats minimum).

From the original questioner:
Epoxy joinery: Using double doves and other traditional joinery methods, leaving them out 1/32 and using West Systems epoxy and graphite additive. The joint will show as a nice black line.

As far as the second post, from what I have read and from what the folks at West Systems say, using epoxy as a top coat will be fine as all of the windows are UV filter, and none of it will be getting direct sunlight. I have furniture 4 years old that shows no adverse effect from the sunlight. One is an end table that gets sunlight for about half a day. Do you feel that the conditions on the boat will be too harsh for the epoxy?

From contributor E:
Just remember that the wood will be expanding/contracting a whole lot more in a boat than in your home unless the boat is on 100% climate control. Seen epoxy joints peel away wood and crack, glue lines shear, you name it. Epoxy as a topcoat will survive in the absence of UV but I always put down several coats of the epoxy followed by several coats of a UV resistant clear coat. The epoxy, if left as a gloss coat, also is not as scratch resistant as some of the top coats available but it will work if kept within its limits.

Traditional joinery will be a big plus for longevity. I used to make a bunch of money replacing factory boat doors made like kitchen cabinet doors. They usually self destructed within five years around the ocean. Full half lap corners with true floating panels worked best for me.

From the original questioner:
In my experience, if the cabinets and all the brightwork are shot with epoxy, formulated for the conditions, and using a 207 Westy additive, the finished product should be almost entirely free of movement as it is encapsulated in cured epoxy. By wiping with acetone before I shoot and before I join (according to West Systems), I should be able to produce an interior that has very limited upkeep. Does this jive with what you have seen?

From contributor A:
I have several questions regarding epoxy. I would think you would have to be very careful spraying it. Why do you wash the glue with acetone before you top coat? Is there a problem with the top coat adhering to the epoxy? I have used it for a sealer on exterior wood such as hand rails and prior to painting with an oil base paint, I thoroughly rubbed the surface with 400 steel wool. Several years back a paint company coated the rails with a top coat and they claimed the paint peeled off because of the epoxy coating. They did not sand them before painting, and I think what was worse, they painted with an oil base paint before the morning dew had evaporated. I am sure as slick and smooth as epoxy is, it would be absolutely necessary to scuff it prior to painting. I usually apply the first coat thinned with acetone for it to penetrate and lightly sand before applying another coat. Any advice that would help me improve my work?

From Professor Gene Wengert, Sawing and Drying Forum technical advisor:
Epoxy deteriorates when exposed to UV light. Hence it is not used normally for a clear finish. It can be used as a sealer if it is then coated with a UV absorbing non-clear finish (paint). It should never be used as a handrail coating, therefore. Epoxy joints in an exterior environment will also fail due to UV deterioration.

From contributor E:
Spraying pure epoxy resin, even diluted, is a very elaborate form of suicide. This stuff is highly toxic when atomized and inhaled. It is also a powerful skin sensitizer. Ask the Gougeon Bros. why they are allergic to their own product! I certainly have to keep my exposure to a minimum or I break out in painful rashes.

The curing process also creates an amine blush on the surface. This stuff is better than Teflon and nothing is going to stick to it. Each coat should be sanded lightly and then washed with at least soap and water (it's water soluble). We usually finished with an alcohol wipe down. Even another coat of epoxy, much less paint, won't stick otherwise.

As to a few coats of epoxy actually encapsulating and moisture proofing the woodwork, my experience is absolutely no way possible. Epoxy can slow down moisture migration, but to stop it completely takes some pretty significant millage.

We tried an experiment with mahogany blocks once. Coated them as per manufacturer's suggestions and then measured moisture content with a pinless meter. Put them in a bucket of water for a few weeks and measured again. They all had absorbed significant amounts of moisture. Cutting them apart with a bandsaw showed the penetration into the wood is measured in thousandths to maybe just over a hundredth. Even the guys doing cold molding have had to back off of their claims of rot free hulls because of "encapsulation." They still experience rot in the right circumstances.

The interiors I've seen stand the test of long term life. Pretty much all used traditional marine construction (like 60 years ago construction) and no epoxy at all! They were usually finished with marine grade varnish and everything was engineered to allow a little movement. Heck, the boat itself is going to flex and move in usage, far more than most imagine. Then there's the vibrations from engines and water movement. It all takes its toll if everything is made rigid.

I still use epoxy for a lot of marine joinery, but I stick with traditional joints and floating panels as much as possible. I also use a lot of marine grade plywood!

From Professor Gene Wengert, Sawing and Drying Forum technical advisor:
For the info of the readers, marine grade plywood means that all the veneers are "B" grade or better. That minimizes any voids, like you would see in C or D grades. Further, marine grade uses an exterior adhesive. The adhesive is the same as used in any exterior panel and in "CD-X" construction grade plywood.

From contributor I:
You should read Danenburg, "How to Restore Your Wooden Runabout". I bet after reading you will use 3m 5200.

From contributor R:
I have worked with plastic for molds, cars, etc. and when wood is covered with the stuff to secure it from water moisture, etc., you have to remember that when something is heated, it expands and contracts when cooled, so air is expanded when the wood is heated and air is sucked back in when cooled. The moisture is small but there is some taken in during cool winter and such, right through the micro holes in the finish. And it is harder to get rid of than to obtain. Oil is the secret to old finishes that worked - they took too long to dry, but they are still the best. Go find an old fence and but some linseed oil on it or some tung oil.