Fibre Glassing over Timber

BlueLancer

BlueLancer

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I am confused over what weight of fibre glass cloth, I should use as a covering over ply deck which in turn will be covered timber decking. There is 25gram, 48gram, 81gram, 125gram, and 200gram. I will be using epoxy resin on the fibre glass, to bond it to the ply. Advise please......
 
A few questions, sorry.

Is this for the boat in your avatar photo? If so, what is she?

And are you referring to the tightly woven fine cloth, or are you thinking of woven rovings (which are much coarser)?
I hope that it is not chopped strand mat!
(You shouldnt use CSM with epoxy, not least because the CSM soaks up the resin like it is going out of fashion - typically 3 : 1 by weight, whereas tight weave cloth is about 1 : 1 by weight)

Is it intended for this cloth and epoxy to play a part in improving the strength of the plywood deck, or will it be effectively 'strong enough' when the teak deck is fastened (glued, screwed, or both?) down to it?

If it is all going to be massively strong already - eg if the plywood is epoxied down to the deck beams, and the the teak is epoxied down to the plywood - then maybe one thin layer of tight weave cloth applied to the plywood might be enough.

But wait and see what Oldsaltoz says!
 
The prob with CSM and epoxy is the binder in the mat is not soluable in epoxy. OK-ish on flat surfaces, but not on compound curves.
Essentionaly not a good mix.
A
 
Need more info.

Is the ply strong/thick enough on it's own to support the deck?
Or is the fibreglass and resin required to strengthen the structure?
How has the ply been secured to the hull?

Glassing a ply deck is not a major task but it is very important that you do it right the first time.

Please advise. It's 5 AM here in Oz and almost out the door and do expect to get back till around 10 PM local time, so no rush, the more details the better.

Good luck.:)
 
Thanks for the input guys, I will try to answer the questions in order:
Banjan sailor,
Yes the avatar is the boat in question, I am replacing rotten plywood decking with new, the idea being to glass the new ply and then apply a new timber strip planked deck ontop. Probably Douglas Fir.By the way the boat is a supposed to be a Morgan Giles designed MB however it may be a Fairlie. History of boat is still a grey area. All I do know is she was ship to shore boat for the Ark Royal Mk III built 1962, powered by 2 80hp Foden Supercharged diesels. I am redesigning the super structure with a cabin fore and aft with a middle open section ,canvas covered, with wheel house.

Oldsaltz,

Deck will be brand new with marine ply screwed down with S/S srews. This will painted underneath with good quality paint, then epoxied on top before application of glass then douglas fir deck planking bonded on top with screws/plugs and caulked between planks. The idea of using the glass is to belt and brace the deck from leaks.
 
hi
Sounds like you are going to have a great yacht. I am no expert on fibreglass
but I cannot understand why you would create a great waterproof deck of epoxied ply and grp adn then fill it full of holes using screws, that does not make any sense, whatever you do some of the screws will let in water, this will then be captured by the fibreglass and the woood will rot and never dry out. If you are going to that much trouble glue the douglad fir don't screw it.
Cheers
 
BlueLancer said:
Deck will be brand new with marine ply screwed down with S/S srews. This will painted underneath with good quality paint, then epoxied on top before application of glass then douglas fir deck planking bonded on top with screws/plugs and caulked between planks. The idea of using the glass is to belt and brace the deck from leaks.
Click to expand...

Arghhhh NO

There is no point in creating a waterproof layer on top of your ply decking then drilling holes in it.

Please don't screw BUT glue the douglas fir deck planking down. Hold things down with lots of plastic bags filled with sand and use starboard or some other plastic to hold the planks apart.

There is some argument as to which is the best glue to use. Some use epoxy and others a polysulphide like 5200.
 
TQA said:
Arghhhh NO

There is no point in creating a waterproof layer on top of your ply decking then drilling holes in it.

Please don't screw BUT glue the douglas fir deck planking down. Hold things down with lots of plastic bags filled with sand and use starboard or some other plastic to hold the planks apart.

There is some argument as to which is the best glue to use. Some use epoxy and others a polysulphide like 5200.
Click to expand...

Point taken Glue it is no screws, I was not looking forward to plugging a vast amount of holes anyway
 
BlueLancer said:
Point taken Glue it is no screws, I was not looking forward to plugging a vast amount of holes anyway
Click to expand...

I would question whether you need to use any glass either. The normal reasons for skinning in glass is either abrasion resistance or stiffening the ply. If you are adding a decorative surface then you only need to roller epoxy onto the ply to seal it.

The problem areas on this type of construction are the joints between panels and particularly the interface between the deck and the hull. Panel joints can be buts over beams in which case they need sealing, but better to scarph or half lap. If I remember rightly your hull is Double Diagonal which should be stiff enough to bond the deck edges to the planking using glass tape in epoxy. If, however it is a flexible structure (conventional planking) then that interface needs to be flexible to allow the hull and deck to move independently. Otherwise the joint can fail and the result is water getting in - not a pretty sight after a few years!

So, on balance you need to decide whether the deck is floating on the hull or rigidly attached. Unless you are using thin veneers for the final finish then glue the strip down with Sikaflex as this allows for the differential expansion between the materials. As already advised screws are a no-no - I would even remove the screws holding the ply sub deck down and plug the holes. The screws are redundant once the eopxy is cured.
 
Tranona,
The deck supports are tapered dovetails from the inner deck beam to the hull top, in a ladder effect. As you rightly remember,the planking is double diagonal ashcroft.So I take it that the deck to hull fixing is rigid.The edge of the deck beam to the hull is capped by a right angle brass strip pinned every 1 inch, a lot of pins.The toe rails fits on top of the strip screwed down on a bed of mastic. The plan is to cover the new deck with GRP so it overlaps the edge of the deck outboard and then when I fix the new combings for the cabins I can lap GRP tothe sides into the GRP on the deck. My thinking is I should not have any problems with water ingress, where the cabin sides join the deck and no leaking between the edge of the deck and the rubbing strakes.
 
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