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Thistle 1040 - Fiberglass sheathing with epoxy


By Thistle1040 - Posted on 29 December 2011

wooden thistle fiberglassed hullThe first step in strengthening the hull was to lay up some fiberglass tape under the seat supports. These have now been sanded and the edges feathered. Next we'll sheath the inside of the hull with fiberglass cloth and epoxy. The purpose in doing this is twofold. Once finished with new wood and lighter mahogany rails, this Thistle will be underweight. My last wooden Thistle (1014) was 30 pounds light and could not be raced until I added six 5-pound dive belt weights distributed around the hull. Thistles must weigh 515 pounds when racing. Class rules allow for the application of fiberglass cloth inside the hull to add weight. You can't just add it in key areas to stiffen selected parts of the hull. It has to be added over the entire inner surface to be legal. That's fine for us because we need the extra weight and we want the other positives fiberglass and epoxy give us, that being a totally sealed hull with added abrasion resistance. The inside of a Thistle is a busy place when racing. A coat of epoxy and even the light weight, 3.7 oz. cloth I'm using will make the boat tougher and easier to maintain. Too many wooden Thistles rot from the inside out when the varnish is not maintained and water in the bilges enters the laminate. Some might say that having fiberglass cloth inside and outside of the 5/16" thick hull makes it a form of sandwich construction and ends up being even stiffer. It's fine to let that idea stay in your brain if it makes you believe more in your boat but in truth, the 3.7 oz cloth is not enough to build up a layer of fiberglass that would add significant stiffness.

lay cloth on dry and smooth the wrinkles outPeople will argue about putting glass on wet or dry. As long as the epoxy saturates the glass and the wood underneath, it does not matter. It's just a heck of a lot easier to position the cloth on dry wood and that's the first step after a good final sanding and dusting. I use 50" wide cloth to have plenty of room and start by laying in the edge along the keelson. Apply epoxy in small batches starting in the middle so you can work towards the ends and get the wrinkles out as you squeegee the epoxy into the cloth. The final coat goes on with a roller which helps to fill the weave in the cloth and give a smooth uniform finish to the epoxy.

Once the cloth is saturated, you won't even be able to tell it's there. That's what I like about the 3.7oz. deck cloth from Tap Plastics. It's just the right weight for this and it works perfectly. Heavier cloth just holds more epoxy. It takes just under 1/2 a gallon to sheath one side of the Thistle hull. I'm using Progressive Basic No-Blush epoxy. It's the best epoxy I've ever used. It does not have an amine blush after curing and it does not irritate my skin they way other epoxies do. Progressive epoxy's owner, Paul Oman is a boat-builder and sailor. He knows all about epoxy but very little about making good web pages. Good epoxy, bad web site but full of good information.

Once the cloth is saturated with resin, all you see is the wood. When the boat is finished, this surface will be sanded and coated with a UV protectant or it will start to yellow. I'll probably use sprayed on two-part clear coat instead of varnish though.

The glass cloth is clear after the epoxy goes on.

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EpoxyUSA.com Progressive Basic No-Blush Epoxy

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