The following article by our Epoxy Advisor, Pete Johnson, first appeared in “Watercraft” Magazine before appearing in The Bulletin and is reproduced here with the author’s permission.
All epoxies are fundamentally the same in that they will seal the surfaces of timber to prevent the ingress of moisture and stick wood and many other substances, one to another, with incredible strength. In their early days they were hailed as a boon and a blessing to wooden boat builders everywhere. Today, the hype has subsided and most serious builders treat epoxies like good tools, carefully choosing those which best suit the job in hand and the way they like to work.
Most epoxies require you to mix two components, although some rare ones require three. For marine use, we tend to stay with the two component type: resin and hardener. The first epoxies to come into mainstream use in the 1970s normally required 7, 6 or 5 parts of resin to be mixed with 1 part hardener. These epoxies revolutionised the way many amateurs built boats, allowing the use of cheaper, less durable timber and plywood which, when encapsulated in epoxy, remained dry and thus prevented rot. Epoxy with the right filler powder added was also an excellent gap-filling adhesive, generally stronger than the timbers it was glueing together and thus very forgiving to less skilled woodworkers. Amateurs could turn out safe and strong seaworthy craft.
There were, however, some drawbacks to these first generation epoxies. Here are some of them.
* Mixing could cause problems. Epoxies must be mixed really thoroughly and failures could result if the hardener was not mixed uniformly through the resin.
* In the mixing process, the chemical reaction could produce heat so fierce that it would melt plastic mixing pots and could even catch fire!
* As soon as the mixing commenced, the epoxy would start to cure and set in the pot, so you had to work very fast to spread it on to the glueing surfaces and get everything clamped up. This often meant expensive waste when some of the mix went off in the pot before it could be used.
* An ‘amine blush’ could be produced by the chemical reaction as the epoxy cured. This covered the epoxied surface with a wax-like deposit which would clog up sandpaper and you would not be able to put on any more coats – of glue, paint, or varnish – until you removed it. The best method of removal was washing everything down with water and in some cases, a dilute acid. In the workshop, this is a nightmare as everything gets wet and you cannot continue building until you have dried everything properly.
* When sheathing the hull with glass cloth, trying to mix enough epoxy to wet out the cloth properly meant you had to mix several small batches at once. This was a problem because those first generation resins tended to be very viscous. You could not get them to go through the cloth easily and often had to resort to physically pushing the epoxy through the weave or it would just lay on top doing nothing.
* When glueing and filleting, some early epoxies needed special additives to make up gluing constituencies.
* Many first generation epoxies had very powerful odours which could be intoxicating – though not in a nice way! – affecting the user very quickly. They could also cause burns if you got them on your skin. Prolonged exposure could make you sensitive to epoxy skin irritation, so much so that you would not be able to be in the same room as the stuff without some sort of reaction.
* Some first generation epoxies, while very strong, had no elasticity, so would crack under impact. It seemed that the less hardener the mix specified, the more brittle it was likely to be. Modern epoxies which require 2:1 or 3:1 mixes have good stretch built into them.
* Finally, older epoxies had no defence against the degradation caused by exposure to ultra-violet light.
So what’s new?
So what has the current generation of epoxies to offer? Modern epoxies are mainly a 2:1 or 3:1 mix, so it is much easier to get a good uniform blend of the hardener throughout the resin, so no more expensive failures or wastage due to inadequate mixing. The heat produced is much less, so there should be no fires, smoking pots or melted containers. The open working time is greatly increased, so there is no need for frantic glueing and clamping with sweat dripping off the end of your nose!
With today’s epoxies, there should be no blushing or waxing, allowing you to get straight on with the next coat, which means no time wasted washing down and no mess. As long as you can still mark your epoxy with your thumb nail, the next coat can be applied. If the epoxy has cured to the extent that you cannot mark it with your fingernail, abrade it slightly to take off the shine and then apply the next coat.
Sheathing with glass cloth can still be exciting, whatever epoxy system you use but with the newer types of epoxy, if you split the work into manageable chunks you will not have a problem as long as you keep the area dust free. With the new generation epoxies, you can lay your cloth on dry, cut it to shape and hold it in position with thumb tacks – drawing pins – and silver gaffer tape; don’t use masking tape, it absorbs the epoxy and becomes messy. Next, mix your epoxy. Then, if you are working on a horizontal surface, pour it on and watch it wick out; use a spreader to help it spread further. On vertical surfaces, use a roller or brush to apply the resin and make sure you pop any air bubbles. After the first coat, you should still see the weave. When it starts to dry it will be rough but while you can still mark it with your thumb nail, roll on more epoxy to fill the weave. Leave it to harden and if necessary, coat it again after rubbing it back. When you are happy with the finish, take off the shine and apply paint or varnish.
And talking of over-coating… Modern epoxies are inert once cured, so you can apply almost any finish. However I have found the best approach is to start with a two-pot system and then apply old-fashioned one-pot when the two pack has cured, abrading the two pack to give the single pack something to grip on. Why single pack? I just find it easier to touch up and it will keep awhile; with two pack once it’s mixed that’s it and what you don’t use you have to throw away. A can of one-pot topside paint can be kept and it’s a 5 minute job to touch up any damage.
Additives such as copper or graphite can now be added to help stop weed growth: you coat the bottom with straight epoxy, then coat with epoxy with your additive of choice. When this has cured, cut back to show the copper or graphite; interestingly, with graphite the dull cut-back finish is more slippery that the shiny uncut black finish.
When glueing with the old-style epoxies, some manufacturers said you had to use their own brand of filler to make up your epoxy paste for glueing. Today’s epoxies are more versatile. The main additives are wood flour – much finer than the sawdust you get from your saw bench – silica and cellulose fibres. They have no limitation on shelf life, so they will keep until needed and they work happily with any of the modern epoxy systems. It’s also worth mentioning that there is a variety of wood flours now available, so you can match the filler to the type of timber or create decorative effects with different colours; try coconut or walnut.
The older generation epoxies would start their chemical reaction just as soon as resin and hardener were mixed, which meant you probably wasted far too much. Once today’s epoxies are mixed there is a delay before they start their reaction which extends to ‘open time’.
In warmer climes than ours, there is much discussion about how much heat epoxy can stand before it starts to fail. With modern epoxies, if you paint your boat matt black and leave it in a marina near the Equator, you may get movement in the surface coating. This may show as ‘print through’ – the pattern of the glass cloth becoming visible – but you will have no worse consequences. To avoid this superficial problem, finish your boat in a light colour to reflect heat, and use it. That’s right, get the boat nice and wet. This heating and cooling ages epoxy and makes it stronger. However, there is a point where it has completely cured and it will just be inert.
Today’s epoxies are great…
…as long as you use them with bucketloads of common sense. Most manufacturer’s produce starter or sampler kits, so you can try them in turn until you find a brand which is right for you. How do you know it’s right for you? First, sniff the hardener! I’m serious: you will be working closely with this stuff, if its odour is powerful and unpleasant that’s what your boat will smell like too. Check the pot times on the label: give yourself time to do the job properly. If the epoxy passes both of these tests, go to a demonstration – and don’t be afraid to ask questions. One question you should be sure to ask is: Do you have to wash the job down between coats? If the answer is yes, ask for a demo… Then draw your own conclusions.
To return to where I began, epoxy is just another good tool in your boatbuilder’s workshop and like your other good tools, it should be looked after. Don’t leave it stored in cold and damp conditions. Use it. Keep it simple at first, learn epoxy’s limitations. Then test its capabilities, they may surprise you.
Pete Johnson (Chinawind Yachts)
EOA Epoxy Advisor