Boatbuilding experience with Epoxy vs other Glues – a 30 year review

Part 1

This article actually first appeared in the Duckworks forum.  Then was run as a set of three articles on Duckwork’s main site.  Michael Storer was the author or all of it (of course!).

I try to keep a hold of the larger view, in terms of seeing what really works for large numbers of boats. Happily, I’ve been involved in racing, the wooden boat revival and cruising boats, including many years doing maintenance and repainting. All in quantity, so it is possible to make some sense of the general trends. Also over, what is now sadly/nostalgically, a reasonably long period.

Racing since 1972. Involved in maintenance and painting as my work from 1981 to 1999. Seeing the changes epoxy made to our lightweight wooden boat classes in Australia from about 1980. Assisting home builders with boatbuilding materials and advice since 1989.

I’ll give some specific examples of things you can see from the numbers.

ENCAPSULATION: Where each piece of wood in the boat is sealed on all sides with epoxy. Not just cold moulded with epoxy between the layers as well

To protect plywood the story is the same as every other wooden part of the boat. It needs epoxy on all sides for the coating to be an encapsulation of the part. If this is inconsistent then the capsule is broken and you don’t get the protection.

TELEGRAPH POLES: It was suggested that coating would not be needed if the boat was just made of bigger bits.

Telegraph poles assume the surface is going to crack, but if the pieces remain firmly attached there’s little change in strength from longditudinal splitting. Same for house frames.,

They do say as long as the crack is smaller than your fist 🙂

So maybe the approach works if you build the boat out of telegraph poles and have pumps 😉

EPOXY STOPS LIQUID WATER BUT NOT WATER VAPOUR – BUSTED

WEST make the argument – no air, no water, then no life – in terms of preventing rot. But how effective is the barrier?

The Sabre class Dinghy in Australia is 12ft long and 90lbs so it hits at the low end of our normal hull weight range of 8 to 10 lbs per foot of weight. There have been close to a couple of thousand. I was involved in sailing for a couple of seasons and helped in boat, foil and building development for about 5 years. I know many people who sail in the class including several serial builders.

Before epoxy the boats would put on about 6 to 8 pounds of weight during the sailing season. That’s with every attempt to keep the things dry and very soundly built with no leaks into the tanks. The hullweight limit is quite low so it actually impossible to build a boat to the minimum hullweight with full epoxy sealing coats. It will always end up about a kilogram (2.2lbs) over.

BUT … during the sailing season the epoxy sealed boat won’t pick up any measurable weight. So you are 4 to 6lbs better off.

Now this weight can’t be just where water touches the hull. Liquid water just doesn’t spend enough time in contact with the hull each week (even with after sailing beers) and the boat is hosed down and carefully mopped and dried. The likely problem has always been inside the tanks with the difficulty of really drying out the air. Any water in the tank is small in quantity and might just wet part of the floor area.

Vapour is the culprit. It is also the reason that not all epoxies are equal. The quality epoxies are “high solids” which means no thinner is added. Where thinner is added (cheap epoxy and CPES the thinners evaporates from the epoxy matrix leaving it porous … on a molecular level. Where the largish solvent molecules evaporate from … that leaves enough space for water molecules to go through the matrix.

As soon as you have lone water molecules wandering through … they are behaving like vapour … not like liquid water any more. Not enough of them to make the surface tension effects really appear.

Anyway .. that’s my experience. Not just one or two boats but over 10 or so classes which still have ply boats over lots of years with total number of boats in wood epoxy probably around the 500 mark

To be more techy, here’s a reference to real tests – note the first sentence – there’s more data on preventing water vapour than there is on preventing liquid water intrusion.

Before anyone goes too far with the statement in the article about how moisture content goes up and up with epoxy as well … remember the samples are fully surrounded by the test environment, but boats are usually dryer on the inside. WEST claims (and they are one of the few to have done extensive testing) that the wood stabilises at pretty much the same moisture content as dry stored timber in the local environment – too dry to rot.

Someone said the coating has to encapsulate. That’s correct. Fittings and fastenings need to be dipped in the dreaded ‘pox before running them in.

If it did encapsulate and water gets in to any extent then an inferior epoxy has been used that was solvent based or not enough coats applied (three coats is regarded pretty failsafe).

Here is WEST doing a review of some of their older boats.

This is very much in line with my experience over boats big and small.

I’ve pulled a lot of polyester/glass off boats in strips and the wood is saturated underneath (not a vapour barrier) – never the case with epoxy … but we’ve always made sure people understand how to use it!

My boat experience comes from the pre-epoxy era of racing dinghies. Initially the boats I owned were second hand, glued with urea formaldehyde or resorcinol glues assisted by ring nails, polyester/glass taped and painted with enamel or varnished.

The average maintenance for most of us sailing such light heavily loaded boats was pretty serious. The first couple of years they were fully watertight, but then a few leaks would start to appear in year three. By year 4 or 5 there would be substantial leaks and the boat would no longer be competitive. The paint would have started checking over the more heavily curved areas.

You could strip off all the paint, rip off the glass tape (polyester either holds on or doesn’t stick well at all – usually a mix of both on one boat), retape it, repaint it. Time consuming, dirty, expensive.

Post epoxy you can be sure of no leaks … ever. Maintenance was down to a light sand every five (or often every 10) years providing the boat was kept out of the weather and the sun.

This is experience over thousands of boats with information shared by various class organisations.

I worked for Duck Flat Wooden Boats in Adelaide from 1989 to 1993. I’ve continued working in the same industry and still have contact with people and their boats from the early days.

Can an amateur do a good enough job with epoxy to get the low maintenance and longevity?

Absolutely.

Over the couple of hundred boats I still know about and the thousands I kept track of for some time – the difference between epoxied and non epoxied boats is chalk and cheese.


Can an amateur do a good enough job with epoxy
to get the low maintenance and longevity?
Absolutely.

Now 20 years later all the boats I am still in contact with are going very nicely in good condition and less maintenance.

Several Goat Island Skiffs from the early ’90s have had light maintenance of the varnish at the 10 to 15 year point. Two pot polyurethane paint over hulls is still immaculate.

I felt that epoxy was too expensive and a bit weird in the early days designing and building boats so my original BETH (design #1 1980) was cut up last year – the paint and varnish (no epoxy) had failed to protect the boat – the outside was a real mess of cracking veneers. She was outside but under a good cover … just the same as the Goat Island Skiffs in the para above.

Sigh – everything I did for work was epoxy coated … but I was a bit too inflexible for my own boats in my early days as a designer/builder.

Part 2

If a quality epoxy (no thinners) and applied properly by a caring amateur builder (as a starting standard) it is very effective.

As far as the other building materials for gluing and coating…

Like I said, I started off pretty well in the non epoxy plywood dinghy building days in Australia. Then I was grudgingly converted to epoxy by working with so many boats and customers and seeing it really worked compared to the previous alternatives.

At one point I would have pretty well said that building of anything that is not epoxy is a waste of time. But I’ve moderated from that position.

The last part of the story has been the success of primarily USA and Canadian builders showing me the value of PL Premium and SOME of the other alternative glues and there’s even a place for latex paint.

For gluing alone of our size of boat, epoxy is still in there because of its gap filling properties. It does make a much more fail safe option.

FAILURES

Failures are interesting. They are strongly statistical in nature, which is why I dwell on big numbers of boats … broad experience. It’s not special to me – people who race plywood dinghies or catamarans share their experience – and all the boats are similar enough in construction and use that you can see the weak points of methods and materials. Like hydrofoiling moths … Theres a few odd hydrofoil boats every decade that perform in one direction only or are expensive, but it took Australian Foiling Moths racing every weekend and once of twice through the week to make it truly feasible in all directions at reasonable cost (at least initially). At the beginning they woudn’t fly in less than about 8 or 10 knots of breeze, so they would shoot ahead in little gusts and then the fleet would catch up again.

Now they can takeoff in about 5 or 6 knots or supernaturally keep foilborne in 2 or 3 knots of breeze if they are up already. Week by week, numbers and sharing information.

Working in boatbuilding and restoration – I’ve always got a raft of questions and have huge respect for someone that uses a range of materials. Florida builder and designer Paul Ricelli is a big favourite of mine – he knows more about alternative materials than anyone. Of course you occasionally run into the odd reactionary in the field that will say epoxy is the devil’s work (on some very occasional days I would agree) but usually they are very anti scientific too and are well behind the experience and information curve.

So … how do I know polyester resin is inferior? Because of collected experience on racing dinghies and the number of people that end up reglassing seams or advertisments for used vessels saying “dry boat” (ie no leaks). Never a question with epoxy … the boat almost certainly won’t leak … and is unlikely to start to leak.

But “dry boat” referring to polyester/glass seams means that some boats built of alternative materials do very well indeed and others don’t. But the strange thing is even though extra care and skill goes a long way … it doesn’t ensure a consistent product in the same way.

That’s also my experience with polyester repairs. Sometimes it will stick like Hades, other times you can tease up a corner and pull a whole sheet off a deck. Sometimes you have sticking and non sticking in the one job!

Back in the days when I was involved in PDRacers a couple of serial builders found that the real costs of using epoxy as the glue only. The costs are very competitive with the real costs of alternatives.

But I’ve changed my cheap range of plans – the OzRacer and derivatives and the Quick Canoe standard and Electric to mention alternative glues and some of the cautions that experience shows makes them work better.


Of course you occasionally run into the odd reactionary in the field that will say epoxy is the devil’s work (on some very occasional days I would agree)

The biggest advantage of alternative glues is that they make it so easy just to “start building”. Down to the big box store, get the bits and start the same day. From that point of view, even polyester has some positive points.

The other reason is a bit mythic, but it is real. The limitations of alternative glues (close fits, high clamping pressures and some limitations in waterproofness). Alternative glues should really set everyone’s mind up to do a very craftsperson like job to minimise the weaknesses. But in reality they engender a much more carefree approach.

This is maybe not so good for the longevity of the build, but it does engender a cheerful attitude of enjoying the building. Squirt some goo out of the bottle and screw away. I’m not putting this down … I actually think it is important to the feeling of success and enjoyment of alternative boatbuilding. People get really excited about it.

Compare with the early paragraph I wrote in the previous missal about the stickiness of epoxy and the strongly procedure based work and the somewhat different feeling. There’s a lot of satisfaction when the job is finished … but not such a carefree feeling on the way through.

Alternative glues are rightfully very popular in the US and Canada, and not so common elsewhere.

There is one reason for that.

In Australia to get a very basic exterior ply for a cheap build would start around $50 a sheet for 1/4″ or 6mm ply and commonly be around the $70 mark. Real Marine ply is not that much more expensive. Timber prices similarly – most of our woods are hardwoods and lighter timber for home boatbuilding is often imported. Look at prices about 5 to 8 times what you are paying in the USA.

You do see exterior ply at around $30 sometimes but it is rare to find a stock of sheets that look vaguely nice.

So you have a larger initial investment. The feeling of building a boat to “see how it goes” or building it with the aim of a bonfire a couple of years down the line is totally absent.

So there is economic pressure to preserve the investment even if it adds the $150 to $250 of epoxy to the build.

We need our boats to last to give value to the higher investment.

It is the same in Europe … or even more so.

I’m living much of the year in the Philippines at the moment. Poorer ply just falls apart and there are two or three brands of good local ply. Additionally the crackdown on illegal logging has pushed the price up about 20% in the last year. You might be looking at 15 dollars a sheet. But a middle ranking office worker gets a quarter or less of the income of someone in the USA or Oz so ply is effectively four or many more times more expensive depending on your income.

So again epoxy is usually used. Epoxy is actually somewhat cheaper here because of bulk imports from China, same with glass. But finding reliable sources makes groups like www.pinoyboats.org highly useful. Epoxy has a high use rate in the Philippines too. Even for their “Bahanka” project – a square raft boat with 2ft sides and dead flat bottom from 9mm (3/8″) ply that doubles as a bed when there aren’t floods to deal with. The function there requires maximum reliability despite that it is meant to be a boat for middle and low income earners as well.

But, I’m getting off the track. There is a place for alternative glues etc, particularly in North America. And in other countries when cheapish sources of timber and ply are available.

Part 3

Actually Chief’s example does solve the mystery of the mast moisture.

Someone mentioned cracks that were repaired and cracks that appeared later.

Epoxy tech only works reliably with wood that is sufficiently air dried or kiln dried. There’s no difference in my book except for “soul”.

In the old days specialist sparmakers would put aside big timber for time sufficient for it to air dry. I THINK the rate was a year for every half inch. Kiln dried has a maximum effective thickness of 2 inches … so anything bigger might not be well dried as well.

That said, I’ve used some pieces that ended up rather wet inside for strip planking canoes with few problems. But if the boat ends up with glass on one side only for some time it might curl up like a dry leaf … as one boat did. We had to rewet it and wrap it in plastic and leave it in the sun.

So any spars of grown sticks will not be adequately dried and like the telegraph poles will tend to get longitudinal splits over time, which don’t really affect the strength until bits start detaching or being easily detached.

Just like telephone poles.

You can put glue into cracks, but my own self imposed rule is if it looks structural you have to take the pieces apart enough that you can really get some epoxy goop in there.


Epoxy tech only works reliably with wood
that is sufficiently air dried or kiln dried.

Like John said, it will last for another generation to have to consider it and you can fix some of the fastener entry points by epoxying the fasteners in.

One thing I learned from a restoration architect. I sometimes go to meet up with a group of friends. Alan is a partner (at least – he might be the founder) of Design5 architects in Sydney. Well known for all types of work but especially maintenance and restoration of landmark buildings.

A common friend at the get together had bought a house, pulled the carpet up and found nice floorboards so wanted to sand and clear finish them.

He asked me what to do and I said one of the hard commercial polyurethane finishes. Do it once and it is OK forever. Like a boat built out of well seasoned wood.

However Alan jumped in and said “that’s a bad idea”. He is a quiet gentle soul so I was a bit surprised he was so direct.

“The hard finish will glue the tongues and grooves together and any movement will mean the tongues are broken off the planks along one side.”

He went on to suggest a particular finish that was wax and oil based.

So maybe there’s merit in “old” tech when you can’t encapsulate the piece. As one of the other participants stated earlier in the thread.

It doesn’t work like that for boats though. We see lots of older plywood boats built in the 50s 60s and 70s in Oz and the bits carefully glued with resorcinol and screwed with woodscrews or using ring nails have broken the glue joint and sprung off the backing ply. For each one that is in perfect condition there are 10 where you can slip more than one playing card between the stringers and ply.

Bless screws and nails with alternative glues!

2 thoughts on “Boatbuilding experience with Epoxy vs other Glues – a 30 year review

  1. Urethane glues vs Epoxy? I have used liquid nails also
    Also Polyurethane as a sealent in place of epoxy, any comments? it is a lot less expensive the epoxy though you cannot glue anything to it.

    • Hi Ray,

      Everything holds pretty well the same. If you check the manufacturer’s directions for Urethane and other glues they are very specific about clamping pressure and the allowable width of the glue joint. The specification for clamping pressure is to ensure the narrow glue join as urethane is not gap filling either.

      The advantage of my background is covered in the article – I saw the transition of hundreds of boats in my direct experience in racing fleets to epoxy from the non gap filling glues. The change is boat longevity was dramatic and instantaneous.

      There are some places where Urethane glues can meet their design criterion – close fits and high clamping pressure – and then they should be as long lived as epoxy.

      It is always possible to find some boats glue with non gap filling glues or glass taped using polyester that are “fine” after many years. But in the broader statistical scheme it was a dramatic and sudden change to the characteristics of the boats.

      Urethane can be marginally cheaper. But Andy Linn (if you don’t know the name do a google search for a swathe of boat articles and experiences) built two PDRs keeping an accurate cost record and found there wasn’t so much in it. It was largely perception based as epoxy had to be ordered in one lump … but it was easy for people to drop by a hardware and buy yet another bottle of Urethane, modified PVA or other waterproof glue.

      The way to think about it .. if building toward the bottom dollar, then every avenue should be exploited. The reason is, if you buy one component at a higher quality level it will be let down by the others. So if you find nice ply it makes sense to spend the bit extra to glue it so there will be zero trouble with that aspect.

      I don’t recommend using contact cement like liquid nails in boat building except for holding down carpets. Again … that doesn’t mean that you won’t get away with it. But it doesn’t mean you will get away with it either.

      Best wishes
      Michael Storer