You will hear people talk about polyester verses epoxy resin. When you get into these discussions there is one question you need to ask the person, "What are the differences between polyester and epoxy resin." If they can not give you a simple answer then they don't know and they are spouting off someone elses opinion. Stay away from vinyl ester resin. Most likely you will not find it in your local boating shop anyway.
Polyester resin has a forgiving temperature range for working with it. The catalyst to resin mixture is also forgiving and is not as exact as epoxy resin. Polyester has a higher shrinkage than epoxy. Polyester has a better UV tolerance than Epoxy. Polyester is less expensive and offers corrosion resistance. Polyester resins are fast curing but continue to cure for a long time. This is what causes the odd smell in your deck hatches. The resin will off gas for a while.
Epoxy resins cure harder and provide a stronger bond with the composite materials than polyester resins. Epoxy resins are less forgiving of temperature and need a fairly exact catalyst to resin ratio. Epoxy yellows with UV exposure. Epoxy is more expensive. Epoxy can bond well to polyester. Some gel coats will not adhere well over epoxy but epoxy over the gel coat during initial layup adheres well.
Most composite sea kayaks on the market will be polyester. Some carbon and carbon kevlar may be epoxy if they have a clear finish so you see the fabric layer then it is a safe bet it is polyester resin.
When patching cracks in fiberglass hulls I like to use epoxy and 2 layers of 3 mil glass in a cross pattern layup. This adds extra strength to the weakened area without to much build up.
On punctures I plug the inside with a small piece of foam with a release material covering it. Then a use a thin sheet of glass and polyester resin on the outside of the hole and keep it lower than the gel coat layer. after it has set I remove the foam plug and patch from the inside with epoxy and glass as above. This ensures that the polyester gel coat will adhere properly.
Almost all production kayaks have an outer gel coat layer for color and UV protection. This is usually the first part of the layup of the mold. Gel coat can be applied by a spray gun but usually the repair kits you will find at a boating or marine supply store will be a paste gel.
Please avoid using epoxy you get from Home Depot this is an adhesive, it will work to seal a crack or hole but it will also be a pain to fix later. Repair kits with pre measured epoxy and catalyst packets can be bought and it is just as quick to do a proper repair the first time as it is to fix the hole or crack with two part epoxy adhesive.
When patching a cracked or fractured gel coat it is best to remove all loose original gel coat. as seen in the photos below.
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| Busted gel coat from a collision with a rock |
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| Area after cutting away fractured gel coat |
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| Polyester gel coat paste |
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| Gel coat applied and covered with wax paper |
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| Gel coat after sanding. Watch out for air bubbles. Cleaned it up and added a little more gel coat and it was good as new. |
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