Composite Kayaks Repair
Composite kayaks are made from layers of reinforcing materials saturated with a resin or epoxy. The reinforcing materials are usually fiberglass or Kevlar, though carbon fiber, Spectra and other high-tech materials may be used. As well, these materials can be blended, either woven together in a single fabric or layered on top of each other.
In addition to these materials, core materials such as Spheretex and Coremat may be used. The outside of a composite kayak is usually covered with a layer of gelcoat – an opaque resin or epoxy material that is used to give color and protect the underlying layers from abrasion and UV radiation.
There are several resins and epoxies that can be used. However, epoxies are quite expensive, so they are not often found in commercially produced kayaks. The most common resin is probably polyester, though vinylester resins are also used.
It is easy to identify a resin by its styrene smell when new; modern epoxies are usually odorless. When repairing kayaks, it is important to know whether you’re dealing with resin or epoxy. A resin repair will not stick well to epoxy, whereas an epoxy repair will usually adhere to anything. If you have an epoxy kayak, you will not be able to use a resin gelcoat – you’ll have to search for an epoxy gelcoat.
An excellent source that describes the characteristics of resins and epoxies is on the web at SPS Systems web site.
In most modern kayaks, the reinforcing material is woven into a fabric. Some manufacturers, notably in Britain, also use a layer of randomly oriented chopped strands referred to as mat, which is easier to form around odd shapes but does not produce as strong or light a composite as a good woven fabric. In British kayaks, mat is often found in the deck.
The core material increases the thickness of the lay-up without as much weight as would occur if more reinforcing layers were used. The increase in thickness results in an increase in strength and stiffness. Core materials are usually used for keels and wide, flat deck areas. These core materials are usually a fabric that is filled with tiny glass spheres or a lightweight, low-density material.
In construction, two female molds are made, one for the deck and one for the hull. The molds are covered with a layer of gelcoat, which, by virtue of it being a gel, sticks to the mold and doesn’t run down to the bottom (at least, not too much). On top of the gelcoat, layers of reinforcing material are added, each being saturated with resin.
Some makers will vacuum bag the lay-up at this point. Once all layers are in place, the entire mold is covered with a thin plastic film and the film is sealed around the edges of the mold. The edges of the mold are designed to hold the film and still allow air and resin to flow under a vacuum. The mold is then subject to a vacuum and the plastic film is sucked down, compressing the layers together and squeezing out the excess resin. This process enhances the saturation of the fabric with resin, reduces the amount of resin to a minimum and results in a kayak that is light and strong. It is cured in this condition.
Other makers will not vacuum bag. Very good quality composites can have the resin impregnated fabric squeegeed until there is a very fine layer of resin over the fabric and no dry fabric or bubbles are present. Done well, this can result is an exceptional composite. Many makers will not do this, as it requires a higher level of skill among the workers. If not done, excess resin is present. The impregnated fabric will not have a tight, compact finish and will be heavier and less strong for the weight.
Once cured, the resulting hull and deck are cleaned up, drilled for fittings and cut to final shape and joined together. In some kayaks, the joint consists of a plastic extrusion, H-shaped, into which an adhesive is placed (Figure 3). The deck and hull are placed into the seam and the inside of the seam is covered with a fiberglass tape to seal and reinforce the extrusion. Fittings are then added to the kayak and the craft is complete. Those without plastic extrusions use a fiberglass tape directly.
Some manufacturers will use an H section that has a hole in the horizontal part of the H. This hole is used to feed the rudder control cables to the rear of the kayak without creating a hole in the hull or deck. Current Designs’ Solstice series is an example.
Working with repair materials.
The most common damage to a kayak is scratches and chips in the gelcoat. These are not too serious most of the time and do not meaningfully contribute to the loss of strength or performance of the kayak. For that reason, repairing a kayak once every year or two is sufficient for normal wear and tear.
If damage exposes the underlying composite to UV radiation, this is more serious. Since resins and epoxies are not UV resistant, removing the gelcoat can result in the underlying composite being damaged by the sun.
Gelcoat repairs are easy to do. They are a little messy and care must be taken, as the materials can be toxic until the job is complete. Gelcoats are a two-part material. Epoxies are usually mixed with hardeners in ratios of two- or one-to-one by volume. Resins, however, use only a few drops of a catalyst. This is often MEKP – methyl ethyl ketone peroxide.
MEKP is a nasty chemical and you don’t want it on your skin. Epoxy hardeners are also nasty stuff and are hyper allergenic (i.e. they are likely to cause an allergy). If you’re a kayaker, you do not want to develop an allergy to epoxy! In addition to this, when sanding, the dusts produced are not something to breath.
Also, the products off-gassed from resins are flammable and have a low flash point – around 32C. No smoking and keep flames away. Make sure the area you work in is well ventilated – I prefer to work outside.
Safety equipment should include gloves and, when sanding, a very fine dust mask. If you’re unsure about the work area ventilation, a respirator with cartridges for Volatile Organic Compounds should be used for the fumes and one with extremely good dust rejection for the sanding.
There are several gloves on the market that are good. Latex gloves are cheap and common. Vinyl gloves are also available and are better for those with latex sensitivities; they’re less stretchy, though, so sizing is more critical. I prefer the more expensive nitrile gloves for their puncture resistance coupled with reasonable stretch. The latter are easily identified by their green color. Unfortunately, nitrile gloves are not recommended for resins, due to the use of MEKP.
|Plastic glove types|
|Use for||Epoxy, resins||Epoxy, resins||Epoxy|
|Excellent for||Acetones, alcohols, analine dyes||Acetones, alcohols, polyurethanes|
|Good for||Dilute acid, ethylene glycol, ketones||Short term use with finishes and glues containing alcohols, linseed oil, mineral spirits, naphtha, tung oil, esters, ethylene glycol, 90% phenols|
|Not recommended for||Solvents, oils||Paint strippers, strong solvents||Analine dyes, ketones (inc. MEK – resin catalyst), Citristrip|
For more information on chemical strengths and weaknesses,
Get a lot of gloves – when working with large amounts of resin or epoxy, it’s a good idea to put on several layers of gloves. That way, as your outer glove becomes contaminated and too sticky to use, just peel off that layer and carry on – no need to stop to put on a second glove. The working time of resins and epoxies can seem awfully short, so this time saver will be appreciated.
Working with Gelcoat
Gelcoat comes in several types. Notably, there are different resins – try to get one that is the same as the kayak. If you’re unsure, contact the manufacturer of your kayak.
Gelcoat can be pigmented to match the color of your kayak. Mixing colors in gelcoat is just like doing so with paint and as such, there are both neutral base and white base gelcoats. Get a color chart for the gelcoat you use and determine whether you’ll need to start with neutral or white base. The pigments are available in little tubes. You usually only add a few drops to get the right color.
In addition to this, you have to decide whether you want a gelcoat that contains wax. Air inhibits the curing of gelcoat. The wax in gelcoat forms a film on the surface that protects it from air while curing. The wax also reduces the strength of the gelcoat somewhat.
If you use a non-wax gelcoat and let it cure in air, the surface will not cure and will remain sticky. This can be cleaned off, but it’s better to deal with it properly. Covering the gelcoat with a release film protects it from the air and allows it to cure. Release film is a plastic sheet – Saran Wrap or similar household food wraps can be used as a substitute. Release sheets also provide a smoother finish when cured, so I prefer waxless gelcoat and use a release film.
Don’t use Mylar – it’s polyester sheet and will adhere to the resin or epoxy. You can also cover the surface with PVA – Polyvinyl Alcohol. This is available from vendors of gelcoat. Don’t confuse it with the other PVA – Polyvinyl Acetate!
Mixing pots and spreaders should be made with a plastic that will not react with the resins. If you want to reuse a plastic container, look for a recycling symbol with “PP” (polypropylene) or “HDPE” (high density polyethylene). You can also cut spreaders from these. Avoid “PS” (polystyrene) and polyesters. I use yogurt cups or cat food tins for mixing. When finished, I let leftover gelcoat cure in the mixing containers and then peel it out. The cured gelcoat can be throw away and the containers tossed into the recycling bin.
Filling Scratches and Gouges
The first thing to do is ensure that the area to be repaired is clean and dry. You can clean it with soap and water, but alcohol, acetone or other solvents can be used on really nasty dirt. Glues and sticky stuff, like the residue from stickers and logos, can be removed with products like Goo-gone. I find WD-40 works well for modestly sticky stuff. Clean the sticky bits off first and then wash off the goo remover.
Remove any loose chips of gelcoat and sand down the area. Sanding will produce a rough surface with more surface area and provide more area for the repair to bond to You can dry the area with a hair drier if the kayak has not had time to air dry. Let it cool off before continuing.
Make sure you’re ready for everything before adding MEKP to the gelcoat. It will cure surprisingly fast once you get going and you will appreciate being prepared. All your tools and stuff should be at hand when you start.
Now mix the gelcoat. If you are coloring it, blend the pigments to get the right color before adding the catalyst. The color will not change when cured. Once you’re sure about the color add the hardener. If you’re an artist, this should be easy. Me? – After trying to mix colors, I prefer white kayaks.
When tinting gelcoat, you can blend a batch that’s more than you think you’ll need. Pour a bit of the pigmented gelcoat into a mixing cup and then add a touch of MEKP. That way, you can work with a convenient amount and if you need more, the pigmented gelcoat is already available and will match. If you finish with excess, store it in a freezer or fridge (well sealed in a heavy plastic bag) to extend its shelf life. The next time you need some, no color matching is required! Don’t forget to bring it to room temperature before using,
Catalyze the gelcoat in a small container. The curing of resins and epoxies is an exothermic reaction. Heat accelerates the curing and the pot can get too hot to hold! Large surface area, shallow vessels will allow the gelcoat to stay cooler and last longer than a deep, small-diameter pot. It’s better to work with small amounts at a time. A couple of tablespoons can easily repair an entire hull’s scratches. Don’t reuse the pot if you are repeatedly mixing colors. The catalyst is not all absorbed into the resin and any residue will activate any resin that you add to the pot. Always work with a clean container.
Use a small spatula to apply the gelcoat to larger chips and holes. A wooden mixing stick, like a coffee stirrer or a tongue depressor, is good. You can get these at most places that sell gelcoat or at a craft store. Spread the gelcoat over the area and try to get it as smooth as possible. Make sure you get any bubbles out. You can also use a small paintbrush if you prefer.
If you are filling long gouges or multiple gouges over a large area, spread the gelcoat through the area with a spreader or squeegee. This will fill the gouges and wipe the gelcoat off the smooth surfaces. Don’t worry too much about thin bits of gelcoat in areas you don’t want – it’ll clean up easily later.
If you are filling a really big hole and the gelcoat it too runny, you’ll have to mix it with a thixotrope. A thixotrope is a thickening agent. Glass spheres are one type, as are Cabosil (powdered silica gel), flour (wood flour or wheat – if you use the latter, do not use self-rising or other adulterated flours). If you use a thixotrope, do not use it to completely fill the area. Instead, use it to fill the bulk of the hole and top it off with pure gelcoat.
The thickening agents aren’t completely waterproof and some, like Cabosil, do not sand well. Covering the thickened gelcoat with a thin layer of pure gelcoat will allow a workable, waterproof surface. For large surface damage right on the keel line near the bow and stern, you could consider using Cabosil at the surface – this will provide more abrasion resistance than plain gelcoat.
Make sure the shape is good, as sanding later will be more difficult. I like to use an artist’s spatula for applying and spreading thickened gelcoat.
An alternative to using a thixotrope is to use many thin layers of gelcoat. This will take more time, but is just as effective. If you’re using a waxed gelcoat, it’s probably a good idea to lightly sand the surface between coats of gelcoat. While you could add a new layer to a partially cured layer, I’d recommend letting it cure fairly well between coats. Partially cured gelcoat is like livered paint – sticky and slippery at the same time and you can mess up the gelcoat very quickly.
Once you are satisfied with the gelcoat, cover it with release film. Pull the film tight to try to shape the gelcoat surface to the final shape. If you use a very thin film like household wrap, you’ll find it wrinkles a lot and can be difficult to work with. Try to get the best surface you can. Stiffer release films can be hard to wrap around tight, complex curves, so household wrap might be better in this case. Having both types of film on hand will allow you to adjust for surface shape.
Leave the gelcoat to cure for at least the time recommended in the product documentation. Make sure the temperature conditions are correct.
When fully cured, peel off the release film. You’ll have to judge at this time whether to mix some more gelcoat and clean up some of the repairs. It’s possible that some gelcoat flowed out of the holes and gouges and needs to be touched up. As well, bubbles that you didn’t see earlier are now visible. If you need to fix it some more, do that now and wait for everything to be cured before continuing.
If you are satisfied with the gelcoat, you have to clean up the excess and sand down the rough and high spots.
Finishing the repair
A sharp knife (X-Acto or razor) can be used to scrape off drips and such. A really big, hard blob can be removed with careful use of a power tool like a Dremel with a sanding drum. I emphasize careful, as one slip will mean a new repair job.
Getting a smooth, glossy finish will require a lot of elbow grease. Several different sanding and polishing steps are required to get the best finish.
Sandpaper is graded by grit number. Not all sandpaper is covered with sand – other abrasives are used, but the generic name is often used to refer to all types. The abrasives are sorted by size. The size is specified with a number that represents the number of holes per inch in a sieve screen used to separate the grains. A grit number like 120 means that there are 120 holes per inch. 600 means that there are 600 holes per inch. As such, the higher the grit number, the finer the sandpaper. Fine sandpapers will remove less material than coarse sandpapers. As well, fine sandpapers will make smaller scratch marks than coarse.
Sandpaper comes in several types. Most are paper backed, some are cloth backed. They can be wet/dry or dry. Wet/dry sandpapers are used for wet sanding and the backing material and adhesives are designed to resist the water or oil used to wet the surface. Wet sanding has several advantages – the most important to me is the reduction of dust. Wet sanding will result in a finer finish than dry sanding with the same grit paper.
The process of creating a smooth, polished surface consists of several steps. The first step is to remove the lumps, bumps and excess material with a coarse abrasive. This levels the surface, but produces a rough finish. The next step is to use a finer abrasive and remove less material and change the rough surface to a smoother, but still scratched surface.
A finer abrasive is then used to remove even less material and reduce the size of the scratches even further. The process continues with finer and finer abrasives until the surface is polished to satisfaction.
Start with a relatively coarse sandpaper. 220 grit will remove a lot of gelcoat and remove a lot of bumps. I usually start with 320 if a release film has done its job. Once you get a smooth surface, switch to finer sandpaper like 600. Then use 1500 to 2000 grit. Any finer finish usually comes from a polishing compound.
Rubbing compounds can also be used. Courser rubbing compounds can remove gelcoat over a large area much like a 320 grit sandpaper but with a finer surface finish. I use rubbing compound to remove the leftover gelcoat that results from squeegeeing gelcoat over a large area. Rubbing with a rag will suffice, but a power buffer is faster.
Once the gel coat is very smooth, polishing will bring it to a gloss. Some polishing compounds made for composite boats contain a wax and will really shine. You can also use some polishing compounds made for automobiles.
If you’ve done a good job on color matching and have polished the gelcoat well, the repair will be indistinguishable from the original.