Epoxy is sometimes viewed as a miracle material for boatbuilding, particularly by amateur boatbuilders. It is a good and very useful material but, as with everything else, it must be used correctly if it is to do its job as intended. Use it incorrectly or for the wrong application and you may have big problems on your hands. This post is primarily about epoxy adhesives, not epoxy coatings.
Epoxy adhesives are fairly thixotropic, i.e. they have a consistency that does not allow them to flow easily. They have gap-filling properties, which means that they have the ability to stay in the joint and fill in the gaps. But that gap-filling has limitations, it can't be counted on to compensate for your bad woodwork. Go beyond that gap-filling ability and the joint may be seriously compromised.
Each type and brand of adhesive behaves in its own way and the differences in characteristics must be taken into account when fabricating anything with them, if they are to perform as intended. Epoxy does not like surface-to-surface contact. There should be a very thin layer of epoxy between the surfaces being joined to get maximum bond strength. With a very thin layer the bond is strong but, if the epoxy layer is too thick it can be brittle and might be easily broken. In contrast with epoxy, resorcinol needs wood-to-wood contact for maximum strength and likes a tight joint. That doesn't mean that you should build sloppy woodwork just to get epoxy into the joint.
I am not the world's best woodworker but I try to make my joints as neat and close-fitting as I can, gluing them with resorcinol rather than epoxy. I take pride in making neat joints with minimal slop in them. That is the way to build a strong boat that will give you years of happy seafaring. I have been reminded of this in recent years when discussing past modification work done on the Didi 38 "Black Cat" with her current custodian, David Immelman. He has told me numerous times that he always knows when he is taking apart some piece of the original structure that I built more than 20 years ago. He battles to take apart my neat and well-bonded work, whereas structure added by others employed by the owner often comes apart rather easily. I have seen that modification work and some of it is rather sloppy. If the visible parts are to a low standard then it is anybody's guess how bad the hidden parts might be.
I occasionally see woodwork that seems to have inexplicably deteriorated or joints that are separating or have come apart. Inspection or investigation can generally come up with explanations or possible reasons for the deterioration or damage.
An example is a boat to our MG30 design, with transom-hung rudder, which was racing and moving fast. Her rudder came off, taking a large portion of the transom with it. In the absence of answers, the discussion about the damage was coming to the conclusion that plywood construction was just too weak to take the loads of sailing at high speed. I was many thousands of miles away but was approached for my opinion. After seeing photos of the rudder I queried a mark on the leading edge, suggesting that it could be whale skin. That is what it proved to be and the investigator found crew from other boats who reported seeing blood in the water after the incident.
|The rudder, with small mark half-way down the leading edge, which proved to be whale skin.|
|This is the groove in which the knee was housed. Again, there is no sign of a bond between the knee and the back face of the groove. The partial damage to the plywood sides of the groove indicates that the sides had only partial adhesion as well.|
The test when dry-fitting a joint that will be glued with epoxy is that the two parts should fit together easily, with one part sliding easily into the other with no pressure and minimal wobble. If you need to use a hammer/mallet or a clamp to force the parts together then it is too tight. There will be no space for the glue that is supposed to make the joint permanent and the glue will be scraped off the surfaces as the joint is closed. Trim a little material off to give a better fit. If there is more than just a little wobble then you have too much space in the joint for the glue to fill. Rocking of the wood while positioning the joint can force some of the glue out of the gaps and leave air pockets, which will weaken the joint.
When gluing two flat surfaces of wood together or plywood against wood framing, you need good, clean and matching bonding faces. A rough-sawn surface might seem good for bonding because it has lots of roughness for the adhesive to grab onto but it is likely to have many highs and lows, creating gaps that will either be filled or left as air pockets. It will have many loose fibres and splinters that will damage the bond if the epoxy doesn't envelope the fibres, to bond them to the base wood as well as to the other half of the joint. The coarse surface will also wick epoxy out of the adhesive, leaving the filler with reduced epoxy and the possibility of a dry joint. Slightly rough is fine as long as it is clean and dust-free. A clean planed surface is good for holding power, with the epoxy able to penetrate into open pores on the surface. Sanding the surface can weaken the bond because sanding dust may clog the pores, reducing penetration into the surface. Blow off any dust before gluing because a dusty surface can destroy the bond in patches. The bond will also be badly compromised by contamination from dirty rags, sweaty hands and even the exhaust gasses from diesel engines running nearby.
A builder recently sent photos of the wing of his Paper Jet skiff, which has some cracking of the epoxied joints within a year of first launch. He had got water into the wings due to a building error and a leaking drain plug. He felt that this had soaked into the wood, swelling it and cracking the joints. But all surfaces inside the wings are sealed with epoxy coatings, so water should not have been able to get into the wood unless he didn't coat those surfaces as instructed.
|This is the outer corner of the leading edge of the wing. The wood-to-wood epoxy joints have cracked.|
|This is the aft end of the wing, where wood-to-wood joints have also cracked.|
I don't think that the cracking in these photos is from wet wood though. Happening so soon after launch and cracking the way that it has, I think that the cause of the cracking lies with the epoxy in some way. There are various possibilities.
1) The epoxy may have been old stock, with the heavier components settled to the bottom and the lighter ones risen to the top. If this happens and the epoxy is used without first thoroughly blending the contents of each can, the resulting mix may never cure properly. Always stir or shake your epoxy containers before you take material out for mixing. This will ensure that you always have all chemicals in the correct proportions, not just those that are at the top of a newly-opened container or the gummy dregs at the bottom of an old unmixed container.
2) The epoxy may not have been mixed properly. Some epoxies mix easily and don't need a lot of attention to the timing nor the process. Others are more demanding, needing more time to mix very thoroughly. I have had personal experience of this when I used an epoxy coating that needed 5 minutes of mixing when I was accustomed to about half that time. It seemed to cure but the polyurethane paint system applied over it cracked after a few months, alerting me to a problem. I was able to peel the whole lot off with the aid of a hot air gun, finding soft epoxy under the hard paint. That epoxy, if used as an adhesive, would not hold the pieces together.
3) Too much filler additive in the epoxy can lead to weak dry joints if any of the epoxy migrates into the wood. A coat of unfilled epoxy applied with a brush and allowed to soak in for a few minutes, before adding a thin film of filled epoxy, will prevent this from happening.
4) Some builders pre-coat all wood before assembling the boat. Pre-coating the surfaces to be joined with epoxy is OK if the joint is completed within a few hours, when the adhesive in the joint can still cross-link with the initial coating to make a chemical bond. Left longer than 12 hours it needs to be sanded to help the adhesive to grab onto the coating but it will be a mechanical bond, much weaker than a chemical bond. It would be better to mask off the gluing areas when pre-coating plywood and framing so that the epoxy joints can be made on virgin wood surfaces.
It takes a lot of time, effort and money to build any boat. It is heartbreaking when things go wrong and repairs are needed to the beautiful product of your work. Don't let careless jointing and epoxy habits damage your work.