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Old 31-08-2015, 16:49   #1
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Glueing plywood to fibreglass.

Well, threw the old boat up on the hard for a couple of months to do a few jobs which has expanded to include replacing all the tired plywood lining the inside of the hull and the bulkheads inside the saloon (thanks, Dear!). The original plywood was glued 30 odd years ago with thickened polyester resin on the hull which was a bit hit and miss but is extremely tenacious where it was a "hit" (probably because the hull was still "green" when done) and thickened epoxy on the bulkheads (which are plywood coated in epoxy). This has resulted in a somewhat uneven surface on the hull for re-attaching the new plywood. Which isn't a big deal because the original plywood didn't follow the curves of the hull that truly anyway and stood off about 1/8" or so on average and the - still attached - resin chunks will act as a spacer to the original positions.

Now I'm looking at reattaching the new stuff which will be 1/8" plywood. I'm thinking of the following alternatives (in my preferred order):
  1. Industrial strength waterproof liquid nails
  2. Flexible tile adhesive
  3. Polyester resin

Opinions and tales of similar experiences welcomed...
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Old 31-08-2015, 17:03   #2
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Re: Glueing plywood to fibreglass.

I would skip the liquid nails and the tile cement and go with epoxy in a tube for a caulking gun. They have the cool mixing spout so it's easy to use.
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Old 31-08-2015, 17:20   #3
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Re: Glueing plywood to fibreglass.

IMHO adding batten under the ply would be best and how it should be done in the first place. The battens might be made of ” or thicker ply maybe 3” wide and run vertical up the hull at the ends of the ply piece and midpoint or more often also. They are often crosscut maybe one half way thru so they bend readily to fit the hull curve. They can be stuck to the hull with anything from marine bondo to epoxy mix or some of the more modern plastic glues in caulking tubes. Some of these marine bonding agents are very strong and might be all you need. Expensive per tube, but fast and easy compared with mixing bondo or epoxy etc. Then tab a layer of glass over them to the hull a few places. You then put your cut plywood on top of those and screw it into the battens. Might skip the fiberglassing with modern glues/bonding agents.
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Old 31-08-2015, 17:26   #4
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Re: Glueing plywood to fibreglass.

After spending a couple of weeks chiselling lumps of resin, I came to the conclusion that the wood veneer is the weakest component so the glue only needs to be a bit stronger than that and, presumably, have some ability to flex. The other advantages of tile glue and liquid nails is they have excellent initial hold which is great when clamping is out of the question. Having said that, I'm not keen to pioneer either of these attachment methods if they've never been tried before!

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Old 31-08-2015, 17:46   #5
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Re: Glueing plywood to fibreglass.

Get some WEST Six-10.





NEW Six10 Epoxy Adhesive
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Old 31-08-2015, 17:56   #6
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Re: Glueing plywood to fibreglass.

I think carefully gluing battens to the inner hulls and attaching the thin ply to this would give you a much better looking product. Thin sheathing tends to follow the contour of the backing and without the backing strips to even it out you may have some pretty lumpy looking paneling.

It is not structural so almost any method that would take the humidity and possible wet from time to time would work fine.
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Old 31-08-2015, 18:02   #7
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Re: Glueing plywood to fibreglass.

Quote:
Originally Posted by minaret View Post
Get some WEST Six-10.





NEW Six10 Epoxy Adhesive
Now that looks the goods, and will be perfect for the bulkheads. Hopefully I can source it from my friendly local supplier. I tremble at the number of tubes needed and the cost to do the sides, especially considering the original glue was laid on with a trowel, however.
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Old 31-08-2015, 18:05   #8
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Re: Glueing plywood to fibreglass.

Quote:
Originally Posted by um saudade View Post
I think carefully gluing battens to the inner hulls and attaching the thin ply to this would give you a much better looking product. Thin sheathing tends to follow the contour of the backing and without the backing strips to even it out you may have some pretty lumpy looking paneling.

It is not structural so almost any method that would take the humidity and possible wet from time to time would work fine.
Don't think this would be needed in this case. Ply wood forms even curves even over a bumpy substrate and, amazingly (to me, at least) doesn't show irregularities. To give an idea of how far off the substrate the ply sat from manufacture, the ply can meet in a neat join at right angles despite the fact that a filleted tab exists underneath.
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Old 31-08-2015, 18:07   #9
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Re: Glueing plywood to fibreglass.

I also recommend the battens but also suggest that you insulate your hull while you are going to all of this trouble. The flexible roll insulation "Armaflex" would be my recommended product.


The links have been changed. This is a better link.
http://www.setsail.com/insulating-boat-hull/
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Old 31-08-2015, 20:22   #10
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Re: Glueing plywood to fibreglass.

A picture is worth a thousand words...

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Old 31-08-2015, 20:34   #11
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Re: Glueing plywood to fibreglass.

Get one of those multi tools with a scraper blade. Get rid of all that old veneer. Fair it with bondo and paint it white
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Old 31-08-2015, 20:48   #12
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Re: Glueing plywood to fibreglass.

Quote:
Originally Posted by Guy View Post
Get one of those multi tools with a scraper blade. Get rid of all that old veneer. Fair it with bondo and paint it white
Lol, I got me two of those things. Best tools ever invented. I can tell you, however, that 30yo resin laughs in the face of multitool scraper blades be they flexible or rigid! Snicker if you must but the method used was to cut each panel down the centre with the multitool and a cutting blade, and then using a small brick bolster (you read that right!) to knock off the old ply by using it as a wedge followed up with various widths of sharpened chisels to take off all the fiddly bits and left overs. I have discovered that the rasp attachment...



.. is brilliant for removing the left over film of plywood from the glue and also for roughing up fibreglass/gelcoat to accept glue. In my case, I had so much to do I resorted to the angle grinder fitted with a 40 grit flap disk. One of the most horrible jobs I've ever had to do on a boat thanks to the amazing volume of itchy and scratchy sanding dust that shoots straight back courtesy of the enclosed grinding areas.

And hence why I'm entertaining thoughts of something less tenacious to stick the new stuff down with - just in case I have to do it again someday.
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Old 31-08-2015, 21:03   #13
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Re: Glueing plywood to fibreglass.

+1 for the multitool (now $15 at Harbor Frt) and +1 for the carbide rasp (under $15 at Lowes). The rasp does wonders with cured epoxy and glass.
The flexible blade on the multitool cuts thru cured silcone and 5200. Money very well spent!!
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Old 31-08-2015, 22:02   #14
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Re: Glueing plywood to fibreglass.

Liquid nails is almost guaranteed to fail. It hardens into a hard brittle state that probably won't stand up to the flexing of the hull. Epoxying battens to the hull and then screwing and glueing the plywood should hold up. You could also use thickened epoxy but would take quite a bit of resin to do the job. DeepFrz's idea of insulating the hull while you are at it is also a good idea. We insulated our Westsail to the water line when we built her and she was warm and cozy without much heat but kero lamps in California and cool in the tropics.
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Old 31-08-2015, 22:36   #15
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Re: Glueing plywood to fibreglass.

Hi, have you considered T-Rex, a cartridge glue that sets up quickly with little to no slump and grips without becoming brittle. Only just discovered it and am impressed so far.
Cheers and good luck.
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