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Mixing Epoxy


While epoxy is a much safer material than many other products used in boat building, some handling precautions should be taken. Epoxy should be stored in a safe manner, so that anyone, especially children cannot ingest it or get it on their skin.
Most people, even those who frequently work with epoxy do not have any health problems associated with epoxy use. A small number of users can, over time, develop some skin sensitivity. This is basically an allergic reaction and has no other consequence. The best way to avoid that allergy is to wear gloves and avoid contact between uncured resin and the skin.

It is recommended to use eye and skin protection at all times when handling epoxy. In case of contact, do not use chemical solvents to remove epoxy from your skin. Plain white vinegar will remove spots and spills.
Uncured epoxy resin and hardener is considered hazardous material, and must be disposed of in an approved manner. Epoxy resin and hardener have a very low flammability, but keep in mind that in a boat building workshop, many of the thinning and cleanup solvents such as acetone are flammable and explosive. Keep your mind on the job. See a physician if any health problems develop.

What is epoxy?

Epoxy is an extremely tough and durable synthetic resin. It differs from other resins used in boat building. Epoxy bonds much better than polyester or vinylester, has superior mechanical characteristics and has a much better resistance to chemicals, water etc.

Epoxy resin, like polyester, is produced by a chemical reaction between two components. Polyester cures (harden) when a small amount of catalyst is added but epoxy cures after mixing relatively large amounts of hardener and resin. Epoxy is the result of the cross-linking between molecules of hardener and resin. The two components combine with each other and the cure of the resin does not produce any residues. When polyester cures, styrene (a gas) is released and creates that characteristic unpleasant smell. Epoxy doesn't smell or only very little.

In boat building applications, epoxy can be used as glue, putty or as a fiberglass laminating resin. In high tech boat building or for larger boats, we may specify different resins for different applications. Viscosity, strength, heat deflection and other properties may vary. In all other cases, we will use a general-purpose epoxy. Our general purpose marine epoxy resin has a low viscosity, wets the fiberglass easily and shows mechanical and chemical characteristics far superior to the polyester resins used in the production of fiberglass boats.

How to use epoxy:

You will never use resin and hardener by themselves. The two components must always be mixed. Our resin is mixed at a two to one ratio by volume: two volumes of resin and one volume of hardener.

Do not try to adjust the cure speed by using more hardener.
It will not work.
Quite the opposite, the resin will not cure.
This is very important: respect the two to one ratio.

How to measure:

We sell pumps that fit on top of the jugs and you can pump two strokes of resin for each stroke of hardener but the easiest way to measure is with a graduated cup.

If you do not have a graduated cup, it is simple to make one.

Use plain clear plastic cups. Fill one cup with water to a certain level: 1", 2", it doesn't matter. Mark that level on the outside and mark the cup with an H for hardener. This will be your hardener measuring cup.
Take another cup and using your hardener cup, fill it with two times the measured volume of hardener. Mark the level and mark the cup R for resin.
Now you have two cups with the exact ratio of 2:1 by volume.
(Dry your cups before using them, epoxy doesn't like to be mixed with water).

Beware of mixing very small quantities of resin. The precision of your measurement will greatly reduce, as the samples become smaller. You should not try to mix less than 2 oz. of resin and 1 oz. of hardener at a time.


Mix the resin and hardener thoroughly. We use tongue depressors but any stick will do. For fiberglass lamination, use the resin without any filler but for use as a glue or putty, different types of fillers must be added. See our video clip of mixing a small batch.

Epoxy glue:

We recommend coating all wood surfaces with mixed epoxy resin before gluing. This is especially important with large contact areas.

To make epoxy glue is a two step process:
- Mix resin and hardener
- Add a filler to the resin, in this case woodflour.

Add enough woodflour to make a paste with the consistency of ketchup, usually 2 or 3 times the volume of resin. See our video on mixing epoxy glue.

Gluing with epoxy is different from using traditional wood glue. Epoxy is gap filling. It needs a small gap to produce a good bond. Traditional wood glues require a tight fit and high pressure. Epoxy requires a small gap and little pressure. You need to keep a minimum of glue between the parts. If you press the epoxy glue out of the assembly either because of a tight fit or too much pressure, the bond may fail. When done properly, the epoxy glue bond is stronger than the parts. Epoxy will not bond properly to a dirty or greasy surface. Clean the surface before applying epoxy.

Let the resin cure. Cure time varies with temperature and hardener speed. See the paragraph about hardener further down.

Putty fillets:

In stitch and glue and in composite boat building, parts of a boat are often assembled with epoxy resin putty fillets.
The putty is made the same way than glue but we use extra filler to obtain a thicker paste. The ideal viscosity is similar to peanut butter.
Putty can be made with wood flour but the ideal filler is a mix of microballoons and silica. Microballoons are microscopic bubbles of glass or plastic (phenolic). Silica is some kind of chemically pure sand.

A mix of resin and microballoons is very light and easy to sand, silica is very hard and difficult to sand but silica is thixotropic. This means that it keeps the mixture from sagging on vertical surfaces. Our filler mix contains the right proportions of silica and microballoons, enough silica to keep it from running but not too much for easy sanding.
Again, before applying a fillet, we recommend to pre coat the parts with unfilled resin.
Often, the putty fillets are covered with one or several layers of fiberglass for extra strength.

Fiberglass lamination:

Fiberglass lamination is the process by which you produce a fiberglass hull or boat part. In stitch and glue boat building, plywood is covered with one or several layers of fiberglass impregnated with resin. After the cure, this will produce a hard fiberglass coating.
In composite boat building, the whole hull core made of plywood or foam is covered with fiberglass and resin, inside and outside.
In production boats, the same process is used to make complete hulls or parts in a mold.

Fiberglassing is easy: a piece of fiberglass is wetted out with resin. The resin is applied the same way that paint could be used, with a roller or a brush.
There are different types of fiberglass. The designer of your boat will specify which one to use. Not all types of fiberglass are compatible with epoxy resin. Some types of glass fabrics are made especially for use with polyester. They use a binder that dissolves in styrene. Since there is no styrene in epoxy, the binder will not allow epoxy resin to wet the fiberglass. Be certain that you use fiberglass fabric compatible with epoxy.
Dry fiberglass is white but as the resin penetrates the fabric, it becomes transparent. Do not use more resin than what is necessary to wet the glass.
A laminate with a high glass content will be stronger than one with too much resin. Too much resin makes a laminate weaker, not stronger. Resin rich is bad for strength and bad for your budget, do not waste resin.

For that reason, as soon as the glass is wetted out (= transparent), squeeze the excess resin out with a plastic spreader. Trained laminators push the resin through the glass with squeegees or spreaders.

Hardener speeds and post cure:

Epoxy resin can cure (harden) faster or slower depending on the hardener you use. The trial kit contains medium hardener, which is ideal in 80% of the cases.
The technical support web site shows a table with cure speeds. Fast hardeners are useful when working in temperatures below 50 F (10 C) and slow hardeners are for temperatures above 85 F (29 C).
Resin will cure faster in a cup than spread out on plywood or fiberglass. The chemical reaction is exothermic. This means that it produces heat. A large mass of resin in a cup will produce heat and cure much faster than resin spread in thin coats. This "pot life" is the limiting factor when choosing hardener speed or estimating the quantity to mix at one time. As an example, a small cup of 6 oz. of resin with medium hardener will have a pot life of around 25 minutes at 70 F (21 C).
After that time, the resin will begin to "gel".
The next phase is called "green" resin. At that stage, the resin is hard like firm cheese but has still no strength. Excess resin and glass can easily be cut or removed at that stage with a sharp knife.
The hard cured stage comes much later: from a few hours to a few days depending on the temperature. Again, with medium speed hardener, there will be no complete cure until the temperature raise above 60 F (16C) for at least a few hours.
A post cure is always recommended: we like to roll our boat hulls out in the sun for a day before sanding or painting.


Boat hulls and parts are usually sanded, faired with a fairing compound and painted. The fairing compound is often epoxy resin with our blended filler.
See our technical support web site for details.

If you did not find the answer to your question, please use our message board and we will respond within a few hours.
Or explore the HowTo files at our technical support web site