What is glass-fibre reinforced plastic (GRP)?
GRP is a glass-fibre reinforced plastic, i.e. a composite material made of fibreglass and plastic. Glass fibres give the plastic special properties, including high breaking strength. GRP is available both polyester-based and epoxy based.
In principle, glass-fibre reinforced plastic can be made from various plastics (such as polyester or epoxy). For a long time, polyester resin was predominantly used in the production and repair of hulls in the yachting sector. In the meantime, an epoxy-based synthetic resin (epoxy) has taken its place in many shipyards and also in refit projects. This more modern material has mitigated many of the problems that exist when working with polyester.
What is Epoxy GRP?
The basic concept involves adding two-component epoxy resins to glass fibres and bonding them together to form an epoxy GRP.
Epoxy resin consists of polymers, which become solid through a chemical reaction when a specially formulated hardener is added, hence the term 2-component resin. Epoxy resin mixtures harden at different rates, so choosing the right product depends on the type of work you will be doing with your epoxy resin.
To produce glass-fibre reinforced plastic, fibreglass is of course needed. The glass in question is pulled into very thin threads or fibres, and it is these threads that give epoxy GRP both its tensile strength and high elasticity. They allow it to absorb large concentrated loads and distribute them over the entire surface. Depending on how the glass fibres are arranged in the cured GRP, different properties result.
For example, parallel-running fibres absorb pressure well and distribute it over the surface. If only epoxy is used, without bonding with glass fibres, cured epoxy would easily break under pressure.
When a yacht hull is produced or repaired, fibreglass gauze or scrim is often soaked in epoxy and then applied in several layers to form a laminate. These layers, which are bonded one on top of the other, increase the breaking strength while maintaining a high degree of elasticity, so you can confidently approach a jetty without having to reduce momentum so as to avoid damage (to your boat).
What are the differences between epoxy & polyester GRP?
Polyester revolutionised boat building in the 60s. With it, hulls could be produced in mass using moulds. It was also a lot cheaper and lighter than steel, and compared to wood, virtually indestructible. Nowadays, GRP is just as popular and polyester hulls on boats are not going to disappear any time soon.
But this doesn't mean the end of road for evolution in boat building. Polyester resins have some disadvantages. In particular, polyester is not waterproof and so needs additional protection from moisture penetration – and since boats are in the water most of the time, this additional protection isn't always easy to guarantee.
This problem doesn't apply to epoxy GRP, since cured epoxy is waterproof. Repair epoxy even cures underwater, which can save your boat in an emergency, but it’s also a good product to work with for normal refits or in the construction of new boats or parts, since it cures without emitting any fumes. This means there is no need for respiratory protection during processing and no long-term emissions below deck.
But epoxy resin isn't 100% perfect: epoxy is slightly more expensive than polyester and not UV-resistant. To prevent it from yellowing and becoming brittle, it must always be protected from the sun with a topcoat such as varnish or lacquer. Furthermore, epoxy is more demanding to work with, as you need to be as precise as possible with the quantities when mixing the resin with the hardener.
Nevertheless, epoxy is more versatile in its application and the same amount is stronger, which means that epoxy structural components or entire hulls can be lighter as a result. Epoxy resin also forms a strong bond with almost all materials and adheres to polyester even better than polyester can to polyester.
The greatest advantage in boat building, however, is that cured epoxy resin has inherently high hydrolysis resistance and is virtually impervious to water penetration. A cured hull repair therefore also automatically provides protection against much feared osmosis damage. In fact, an essential part of osmosis prevention on older hulls is to apply a layer of epoxy-based GRP, which can be further improved with a special barrier layer.
1 Fiberglass scrims
So-called scrims made of fibreglass are more flexible and much easier to work with. In its raw state, fibreglass scrim cloth feels almost like textile fabric, very similar to silk. To make it, continuous filament yarn is laid parallel and fixed in an open mesh construction. This gives this composite material much greater elasticity so that it is able to withstand loads, and is ideal for reshaping and strengthening curves or other shapes.
2 Unidirectional fibreglass scrims
If all fibres run in a single, parallel direction, this is called unidirectional glass fibre fabric (or mono-axial). This type of fabric must always be laid across the area where load transfer is expected. This is because unidirectional fibres' strength is in one direction. For a strong repair in all directions, uni fabric should be applied in multiple layers in different directions. This makes sense when making new structural components, as high strength can be achieved without adding unnecessary weight, which is especially important on the hull. Scrims can be bought ex works that are already made and sewn with several layers. However, for easier repairs, multidirectional fabrics are easier to work with.
3 Multidirectional fibreglass scrims
Multidirectional strength with easier processing and less glass is achieved by interweaving the glass fibres in different directions. Thisese glass fabrics isare extremely versatile and usually the first choice for repairs on board, which is why it is also included in most boat repair kits and should be on every vessel. It can be used for repairs on board that are strong in all directions in minutes.
Can I use carbon fibre instead of fibreglass?
Fibreglass is heavier than carbon fibre, so if weight is an issue, it's worth considering using carbon fibre fabric. The carbon fibres used in it have similar properties to fibreglass, but a considerably lower weight. However, carbon fibre is significantly more expensive and with regard to breaking strength, slight compromises have to be made. Consequently, carbon fibre is a much more popular construction material in the regatta sector.
Can chopped strand mat (fibreglass mat) also be used with epoxy resin?
Fibreglass mat is used with the solvent styrene in the polyester resin. Epoxy cannot dissolve the coarsely brittle structure of the mats and cannot soak through. Epoxy resin used with chopped strand mat will therefore only bond on the surface. So when using epoxy, it's better to work with woven glass fabric or glass scrim, as epoxy resin does not form a good bond with fibreglass mat.
Working with epoxy
As mentioned earlier, the reaction during curing of epoxy resin and hardener starts immediately after mixing. Care is required when mixing, since for resin and hardener to react with each other, they must be mixed precisely.
It's important to have the right mixing ratio of epoxy resin and hardener to achieve the best result. Too little hardener leaves sticky soft epoxy, too much hardener remains unbound in the material and makes it brittle. Strictly speaking, a balanced ratio is needed for the reaction to take place, in practice, however, there is a bit of leeway. Depending on the manufacturer, the mixing ratio of epoxy resin is given by weight or by volume. It is worth using a kitchen scale to measure the weight.
However, the easiest way to deliver the correct amount of epoxy resin and hardener is to use pump attachments (such as those offered by West Systems). These pumps are designed to be attached to the resin / hardener container, so that with each stroke the correct amount comes out.
Note: Evenly mix the hardener into the resin immediately using a wooden spatula, and try to keep the mixture as free bubbles as possible. You'll notice the chemical reaction as you stir, increasing heat. If you wait too long to mix or stir too slowly, it could result in too much heat being created in the reaction process. If this happens, a solid lump will suddenly form in the resin and the mixture will then be unsuitable for use.
This heat development can even result ingo as far as spontaneous combustion,. sSo never leave mixed epoxy resin unattended anywherein the boathouse. If the mixture becomes very hot or even starts to smoke, place it on a non-flammable surface, preferably in the open air, at some distance from flammable objects. Flame development is rare, and if so, usually only for a very short duration.
For small repairs, where only holes or small cracks need to be filled, there are also two-chamber syringes are available.. They are often found in GRP repair kits. They produce the exact mixture automatically when squeezed. The resin is also often a little thicker, making it easier to spread without dripping.
Doing repairs with epoxy
For repairs to the hull or deck, epoxy with glass fabric is almost always a good starting point. It doesn't really matter whether vinyl ester or polyester was used, as epoxy bonds very well to all materials. Contrary to popular belief, it actually sticks better to polyester than polyester repair kits or similar.
If there are only small scratches in the hull, damage can often still be repaired by a good polish or paint touch-up. If the damage to polyester hulls runs deeper, however, it is worthwhile using a repair gelcoat with the appropriate colour. (siehe: see: Polyester guide). It's only when damage has affected the laminate that a major repair to the boat is due.
How do you repair laminate breaks and holes with epoxy?
In the event of a very hard impact or if your boat runs aground, it could push the elasticity of the GRP beyond its limits, and result in damage inside the laminate that is not visible from the outside. If this happens, the GRP layers glued together detach from each other or individual fibres break away from the resin mixture. These breaks in the hull can only be detected by sanding off paint and, if necessary, GRP where the damage is. The underlying GRP should exhibit a yellowish glassy texture. However, if powdery white areas are found in the GRP, the laminate is weakened and must be repaired. To do this, sand a funnel-shaped depression into the hull around the damage using an eccentric sander and coarse sandpaper.
When GRP is removed it must be replaced and not just filled with putty, as the hull would remain weakened at this point.
Caution: Although epoxy does is not harmful to inhale, direct skin contact should be avoided. Glass fibres can also cause your skin to itch for days. When working, always wear long-sleeved clothing, preferably overalls, gloves and protective goggles.
How to make a laminate layer with epoxy (explained step by step)
- Cut the required fabric pieces (if necessary, several layers in ascending size, depending on the depth of the ground-out depression).
- Place the fabric piece on a foil and then mix enough resin and hardener so that the fabric can be soaked with a roller brush.
- Soak the fabric completely with a roller or brush. Soaked fabric will become slightly yellowish and translucent. Any areas that are white and opaque will not have been sufficiently soaked.
- Stick the piece used for the repair onto the area, making sure that itthe area is free from dust and roll it down over firmly with a paint roller. The best type of roller to use are velour rolls, as foam rollers react to epoxy.
TIP for working overhead: A viscous mass can be made with silica (see below) and epoxy resin. Before gluing, smear the area of the hull you are working on with this and press the fibreglass fabric that has been soaked in epoxy resin firmly in using a roller brush or, even better, a disc roller. This will prevent the layer from coming loose before curing which could lead to air pockets forming underneath.
- It's better to avoid smoothing or pulling with your hands. Both destroy the structure of the weave, or roving and lead to weakening. Accidental wrinkles can easily be smoothed out and sanded away later.
- Air pockets in the repair area can be removed with a grooved roller or venting roller. This is especially important for larger areas. To do this, roll over the fresh layer of laminate, applying a lot of pressure.
- If several layers of epoxy GRP must be applied, it is best to work "wet in wet" without waiting for previous layers to cure. However, no more than 2-3 layers (or 2 cm layer thickness) should be applied.
- Excess material should not be trimmed with scissors once it has been soaked in resin and hardener. Protrusions are easier to remove with a saw or knife after curing.
Caution: Cured epoxy resin on tools can only be removed mechanically by sanding. It is worth having acetone and a roll of paper towels or disposable rags ready at hand when working. Anything that accidentally comes into contact with epoxy should be cleaned immediately. Change gloves between each work session to avoid spreading epoxy onto tools and the surrounding area.
How to use peel ply
For larger jobs, laminating sometimes cannot be done in one go.. If work is paused and resumed laterthis were the case, the surface of epoxy resin would then hardens to a very smooth surfacely andd would first must have to be roughly sanded again before further layers can be applied. Your entire work area must then be cleaned of dust again.
To avoid this, before longer breaks, it is worthwhile applying a layer of peel ply to a freshhly repaired laminate layers and use a roller brush or grooved roller to smooth out any air pockets. The next day, the peel ply can be removed by hand, leaving a coarse honeycomb structure on the surface, onto which further laminating with fibreglass fabric, or fibreglass scrim, can then be carried out without sanding.
Using epoxy with filling agents to make putty for filling
Epoxy resin is also suitable as a base for many fillers. With appropriate filling agents, it is thus possible to make multi-purpose filler for further work on board. The possibilities for use range from temporary sealing of larger scratches to skillfulskilful modelling of curves and edges on deck, to or repair adhesive, which can be used in an emergency to glue on an entire bulkhead or even a broken seacock. The same mixture of resin and hardener is always used as the basis.
So-called epoxy fillers (filling agent) act as binders and are added to the resin-hardener mixture during stirring in the mixing cup. Depending on the amount of filler added, this results in a mouldable mass, whose viscosity also depends on the amount of filler added.
The type of epoxy filler determines the final strength and weight of masses produced in this way. The following applies in principle: The smaller the particles of the filler, the harder the putty! On the other hand, larger particles are usually lighter.
When silica powder is added, the resin quickly becomes tough and the resulting putty is then very hard. Silica-enriched epoxy is an ideal adhesive for on-board repairs. It can be used to glue bulkheads on the hull or for shelves in the lockers, b. But also for filling large scratches on the hull. In terms of hardness, it is not significantly different from pure epoxy.
Silica itself is a very light powder and is usually sold as flakes in a pot or bag. Accordingly, it can be measured out well and a lot of it is needed to produce a viscous mass. At the same time, however, the resin increases only slightly in volume and structural components made of silica and epoxy resin are correspondingly heavy.
Consequently, with larger shapes, there is a risk that the mass will collapse under its own weight during the first phase of curing.
When it comes to making shapes and structures, so-called microballoons are better here. This product also does exactly what it says on the tin: small balls are mixed with epoxy to create a more voluminous paste.
This filler has a much lower density than silica-thickened epoxy and is many times lighter with the same volume and does not collapse.
Another advantage of adding these small beads is that the lower density results in it being less hardness. Microballoons are therefore ideal for building up larger curves, coamings or similar. Any shapes made can easily be worked on by hand and with sandpaper, so it's a good idea to add a thin layer of fibreglass roving to strengthen corners, especially if they are impact edges.
Smoothing the repair area
It sometimes feels as if a boatbuilder spends half his life grinding in vain. This is because unnoticed unevenness often comes to light during sanding and then has to be filled with new filler. The smaller and faster the grinding tool, the greater the risk of causing new irregularities during grinding itself.
The final stage of repair with epoxy GRP is therefore usually the most lengthy. Some people are fortunate and find their ‘Zen’ in in alternating sanding and filling. But there's also a trick to speed up the process: Wrap a large sanding block in emery paper and move it over the repair area in even strokes with a little pressure. The movement will automatically follow the shape of the hull. This way you can be sure of a smooth transition from hull to repair area.
Epoxy resin must be protected from sunlight
Theoretically, this surface can then be painted directly. Epoxy does not need a gelcoat as it is already inherently waterproof. However, if a polyester hull has been repaired with gelcoat, it is worth applying an epoxy-based barrier coat to also protect the areas around the repair site from moisture ingress. For the final finishing coat, use either a combination of primer and varnish or 2-component varnish without prior priming.
Epoxy-based resin and fibreglass fabric should be in every ship's repair kit. Complete repair kits from well-known manufacturers such as Yachtcare, Seatec or West Systems are available. These repair kits usually contain mixing cups and gloves. It is also worth practising working with the epoxy mixture. There are many opportunities for this, . Bbecause almost everything that breaks or needs to be be put together fixed to something can be repaired or done with epoxy. The bond is then permanent and immediately waterproof. Epoxy just needs to be protecting from sunlight (UV radiation) with a varnish.
Author Hinnerk Weiler
Hinnerk Weiler is a sailing journalist, long-distance sailor and real "old salt". An experienced sailor and expert in boat technology, when it comes to epoxy, he knows what he's talking about.