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Old 11-08-2005, 20:38   #1
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through hull backing block

It is my belief that roughing the interior surface of the hull and backing block and then "glassing in" a marine grade plywood block with resin and cloth is the best installation. However, i dont have experience yet on real world situations where you may have to start with a wet bilge area. I have seen many backing blocks installed with marine grade sealant which may seam like the better approach on a damp bilge. So comments on glass vs sealant are appreciated as well as exterior prep. Do you only have to remove bottom paint or should you also rough up exterior gel coat before "bedding" through hull??
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Old 11-08-2005, 20:46   #2
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I used sikkaflex for the plywood bearer

If I am adding a plywood strengthener and glassing it in, then I always roughen the gel coat first.
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Old 11-08-2005, 20:57   #3
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The major reason of "Ruffing up" the gelcoat is to ensure a good "Key" to add adhesion.
The major advantage to Ruffing up the gelcoat is to ensure a clean surface to adhere to. Especially in a bilge, it's so much the water, but the contaminants that have floated in and on the water. Like oil films. It takes very little to penertrate into the surface pores and then become like a realease agnet. So scuffing away the surface ensure any possible contamination is scuffed away as well.
Just before you place down the backing plate with what ever you are adhereing it with, dry the gelcoat surface with a hotair gun. If the surface is really wet and that can't be done, then don't stick the block down at all. You will trap too much water underneath, even if it is a water resistant adhesive. Infact, you will have better success with mos epoxies at dealing with water, then you will sealants, when used in this situation. A sealant can and will trap water within itself and fail to sure properly. Remember, a sealant requires air to cure. Epoxy does not.
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Old 11-08-2005, 21:20   #4
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I have gotten away from using plywood backing blocks all together. I think that a localized epoxy/glass thickening of the hull that tapers out toward the edges is a more permanent solution.

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Old 12-08-2005, 01:52   #5
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Use Starboard

Then you never, never have to worry about rot or termites eating the wood. Can get wet so what. Found many new boats at the shows using this method. Makes good sense.
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Old 12-08-2005, 05:56   #6
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Wheels. Great point about resin actually performing better if moisture ever was present. (bottom line,remove all moisture) I was not aware of Sikkaflex sealants but they look very good from literature. Talbot, do you use the 291 or 292 ?? Also, on the EXTERIOR gel coat, one should also lightly scuff to remove any "mold release" agents, correct??
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Old 12-08-2005, 06:51   #7
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Sikaflex are very good products. Expensive, but very good. They are a big company that specialise in adhesive/sealants of all types.
Another one to look for is Simpson or Samson, can't remember which one now. But they also make a product similar to the adhesive sealant Sika product. The real key to both the products is the primer. If you want them to realy adhere, then the primer is a must.
I used to do a sales gimick in the chandlery shop. I had a small sample of aluminium stuck together with a 1/2" or so of the sealant. I used to say to customers, if you can pull the pieces apart, I'll give you a free tube. Well no one managed to do it. I even had one guy with vicegrips on one piece and the other in a bench vice and he still couldn't pull it apart. It was great advertising.
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Old 12-08-2005, 11:14   #8
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Cant remember the last time I used sikka as it was several years ago. I tend to use a different sealant for stuff I might want to take apart later . The other thing I dislike about sikka is that whatever is left in the tube after a job is invariably wasted, yes you can stab a hole in the tube to get a bit more out, but I guarantee that more than 50% will be wasted. Whereas the Wurth adhesive sealant does nearly as good a job above the water level, is not so much of a real pain to remove, and I have still been able to use stuff in the tube 6 months later - the seal on the end of the tube seems to prevent further hardening of the sealant.


With sikka, it is important to get the correct item for the job, for example they make one that is much better at UV resistance and is designed for holding windows to the GRP - in fact theere are some people who use this particular glue to be the only fastening for their windows to GRP (i.e. no through-bolts/screws etc)
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Old 12-08-2005, 11:40   #9
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Sikaflex I would be wary of, I did a job of about 2000' feet of gutter and sealed it with this product, it failed and leaked very badly. They did replace there product free of charge , but I had to eat the labor and the job was never right because it was hard to get all the old stuff off.
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Old 12-08-2005, 17:41   #10
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I’ve sometimes combined Jeff’s ‘tapered build-up’ method with a PVC sandwich (core).
Here’s the ‘short’ version:
Apply a layer of FRG cloth (or several), then lay in a piece of 3/8" thick sheet PVC c/w beveled edge, then overcoat with several layers of cloth. Working from hull inwards, each successive layer is smaller than the previous, creating the ‘taper’.
The PVC core is a rot-resistant 'short-cut' in the "thickening' process.
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Old 12-08-2005, 19:59   #11
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RAM, I suspect that you were using the wrong Sikkaflex if the gutter failed. That which is normally sold for the marine industry is very sensitive to UV light and in a gutter sealing position this would probably be the problem. The do sell a special one for sealing acrylic/polycarbonate to GRP, which has a special UV stabiliser.
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Old 12-08-2005, 20:34   #12
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Plus, as I said in my earlier post, YOU HAVE TO USE the correct primer. The Primers are essential to the adhesive ability of the sealant to the substrate.
If used correctly, that is, the correct Adhesive/sealant AND the correct primer, there is probably no other product better for the job. The negatives are, You have to get the product right for the job, There are several ranges of product and several primer types and the combination becomes darned expensive.
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Old 24-08-2005, 23:40   #13
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After looking at the existing plywood backing plates on my 20 year old boat, I wasn't very impressed, so I used a variation of Jeff's method: bought some 1/2" fiberglass plate at a place that sells fiberglass structural stuff, shaped it with a hand grinder with a sanding disk, used a hole saw to cut the hole, and then glued it in place with thickened epoxy, using the thru-hull to hold it in place and aligned. Be sure to wrap the thru-hull with something to keep it from getting glued in asl well!
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