Epoxying techniques (article 2)

Mixing epoxy

The most important aspect of mixing is the ratio of resin to hardener. Usually the recommended ratio is given by the producer, as is for our own glue. For the West System 105 resin and 207 hardener it is 1:5. In the long run you definitely want some pumping system that enables you to achieve this ratio automatically. Firstly it saves time, secondly glue and most importantly – money. Investing into dispensing pumps is very much worth the value. Of course you still need to mix the mixture to acquire a homogeneous mix. This is also true when you add some filler to the base mixture. When adding filler, extra care needs to be taken that all chunks of filler material get mixed, these will otherwise leave a weak spot into the work.

Mixing tools

What we use to mix epoxy are little plastic jam jars, maybe the size of a coffee cup. They are elastic and yet rigid enough to stand the strains of mixing larger amounts of dense filler mixture. We tried with thin plastic drinking cups, but they break when mixing anything denser than a base mixture of resin and hardener. I personally dislike them, because even when mixing clear resign it feels like you are holding something paper. Not solid enough. Besides, the jam jars are reusable. You can just crunch the dried epoxy out, no problem. use a stick to get the little dried buggers off the wall of the cup and you are all set. Maybe you can buy these in larger quantities from some producer or home appliance store, I bet you can. I got ours from my mother who eats the right kind of jam.

Mixing sticks are always missing when you need them. I recommend making many and setting them aside for later use. Depends really how elaborate you want to be. What we have done is cut long strips of wood with a saw and then split them into sticks. Works good. When mixing a dense filler mixture it is good to have a sturdy stick at hand (think left at will).

Plus, you always want to wear gloves when dealing with epoxy. This glue has such volatile moods. If there is the remotest possibility of it landing somewhere where you do not want it to – conciser it already landed. Take the time to wear gloves, change your daily clothes for working clothes, wear a hat, protection goggles, the full load of protective equipment. Epoxy is very toxic!

Mixing very large patches of epoxy could be done with a mixing bit and a drill, but we have not yet had the need for such work.

Applying epoxy

a) Laminating – for laminating it is best to estimate the amount of epoxy you need, prepare it, coat both surfaces and press together with not too much force. You want to see some resin dripping from the joint, this means that all space has been filled in the laminate. Do this when the working surfaces are horizontal, it will give best results. Vertical lamination is also possible, but then the application of the glue will be difficult. Apply the glue with a spatula. Best if you use a glue-comb. These give you a even coat all across the surface, which is important. We use straight edges spatulas, which are better for applying glass. Another option is to use cheap paint rolls. These will suck up some glue, but when coating large areas they are very beneficial, especially for clear coating. Be sure that the mating surfaces are smooth. Uneven surfaces will gulp more glue and leave a worse joint in comparison. When epoxying large surfaces we mix the glue into a cup and then pour the lot onto the surface, eg. when applying glass, then spread it around with the spatula. Be generous with the glue, not too much, but you want to see some glue dripping from the compressed joint, as mentioned above. This applies to all of the following descriptions.

b) Jointing – depends on the surface area. If you are scarfing large surface areas, then refer to the methods above. When jointing smaller surfaces, then we have used a stick to scoop the mixture out of the mixing jar onto the surface and then used the same stick to spread it around. It is good to mark down the area onto which you need to apply the glue with a pencil, for example when applying reinforcing bars to plywood bulkheads. Place the bar on the bulkhead, trace it out, apply the glue onto both surfaces in the right place and apply pressure. This will waste less glue.

End grain of any wood will take more glue than the surface cut in line with the fiber. Precoat these surfaces twice before compressing. If the surfaces are uneven and are hard to smooth, then add slight filler to your glue to make it denser. This will ensure a better joint. What is actually recommended as a standard procedure by the Gougeon brothers is that you coat both surfaces with a clear coat before applying glue, then add the “working layer” which will have some filler in it. This will give the strongest joint.

c) Filleting – is a heavenly fun. This is best done when you have prepared a generous amount of corners to fillet. We use a method recommended by Doug Jackson from www.svseeker.com. Obtain some strong plastic bags with clear no-fold corners. Folded corners also work. Cut a sealed corner off the bag so that it forms a cone. Put the fillet mixture into the cone, wrap it up from the end like a candy and cut a hole into the tip of the bag. Then apply the filler to the corners you need. The size of the hole you cut determines the speed of releasing the glue. This is a good method when you need to apply long fillets. Smooth out the rough fillet with a suitable radius round plastic scraper. We cut ours from the lids of the jam jars mentioned above – very good to have. If you plan on glassing the fillet, then we recommend applying the glass tape immediately after filleting. The tape will adhere to the fillet and you can do some additional smoothing as the tape will make a nice slippery surface to level with your gloved finger. Then apply a clear coat to the glass to soak it trough. The glass has to become see trough unless you are using Kevlar, carbon fiber or the like. A friend of ours used the method of actually applying the filler to the tape and then applying the filleted tape to the corners. This will also work, depends on the situation. We use cheap brushes to apply the clear coat to fillets. We cut the length of the brush down to about 2/3 so that it becomes more rigid and enables more of a forced coating action. This will save you glue, believe it or not. Try to buy these brushes in bulk, if possible. We found some that cost under 1 €.

Fillers

Fillers come in a variety. Too abundant of a variety to do an overwhelming description of them all with our limited knowledge of the matter. We bought a low density filler from West System, they call it microbubbles I think. When dried, this leaves a fillet which is easily sanded down. Depends really on how much you add filler. The denser the mix, the easier the sanding with this low density filler. Later, when we ran out of this low density filler, we started filleting with a mixture of little glass microbubbles and fine cotton dust. Cotton dust actually makes epoxy harder. A cotton based filler fillet will be very hard to sand, it will be very strong. On the other hand, all-glass fillets are very-very easy to sand. So it really depends on the application of the fillet to determine the contents of the mix, using both at the same time is no problem. We have experimented with wood dust left from sanding. This is good for glass-filleting, but not for smoothing out surfaces, because the fibers form chunks and it will be hard to smooth out. When you apply a glass tape to the wood-dust fillet, then the uneven surface of the fillet will not be a problem, since you can smooth it out once the tape is on, as described above. Wood dust filler left a very hard fillet as well. A pain to sand, but definitely cheap.

Additional comments

1) Looking at the Volvo Ocean Race videos i once saw mechanics repairing one of the yachts in a cold shipyard. They used blow driers to warm up the epoxy to make it run better. I had read about this forehand as well. Since epoxy is a polymer based glue (it forms a plastic) heat stretches the molecular connections and makes the glue flow better. We used the blow drier to help soak very thick glass, which took ages otherwise. You can also soften solid epoxy with heat. Heating up curing epoxy is not something which is recommended  but we have seen it work. This is an SOS alternative, which will leave a weaker joint and we can give no guarantee for no-failure.

2) Oak is not as easy to epoxy as fir or pine since it is slightly acidic and very dense. What is recommended is to use a clear mixture on rough-sanded surfaces (what we have done) to pre-soak the maters. Then apply a coat of glue with slight filler content.

3) Always use a dust mask when sanding dried epoxy. You will otherwise develop allergies over time. Try to cover any exposed body parts, it is seriously harmful stuff.