Glass Sheathing New wood - Methods?

Neville220

Neville220

Well-Known Member
Joined
19 Aug 2006
Messages
75
Location
East Bergholt. Ipswich, Suffolk
Visit site
Hi Knowledgable ones,

I need to sheath some rigid new ply wood in GRP.

How would you go about it to prevent the GRP peeling off in future?

I am using polyester resin as I have loads of it.

I really want to get the resin into the fibre of the wood.

There are many ways to cook an egg! So I would like some variations on a theme and if you can say why that method would be strongest that would be great, also and whether it has worked or didn't work for you.

I know I can depend on the depth of experience that is out there.
 
How would you go about it to prevent the GRP peeling off in future?
Click to expand...
Use an epoxy resin rather than polyester One sticks well to wood , the other doesn't!
 
I agree 100% Vic problem is I don't have any Epoxy which I would much prefer as it is quite straight forward, but is quite expensive for a load, however I do have 2 gallons of polyester resin -which is free!

Someone did mention in passing about thinning the resin down so it soaks into the wood - any thoughts on this?
 
Last edited:
MrCramp said:
If you want to do the job properly, so you don't have to do it again take Vic's advice and use epoxy resin.
Click to expand...

I know very well what you are saying is true, and I would be saying the same thing to someone who had posted this, and I might have to bite the bullet and go for epoxy, but I am looking for any possible alternatives.

The foreman at the factory where I get this polyester says it sticks like the proverbial to a blanket but they don't use it for marine purposes.
 
If you must use polyester then abrade the wood very roughly raisng the grain as much as you can.Still not a good idea as it will eventually delaminate.
 
Are you planning on using (say) a layer (or 2) of chopped strand mat (CSM) to sheath your plywood with, or do you just want to coat the plywood with resin in order to try and protect it?
Either way, I would say forget it, go for epoxy instead, and keep your 2 gallons of polyester for another project.
Bear in mind that a layer of CSM will soak up around 3 times its weight of resin.
And a layer of woven cloth (like the stuff that you can buy as a reel of tape in varying widths for doing joints on dinghies and such like) will probably soak up resin in a ratio of perhaps 1:1 (or less).
If the epoxy costs (say) 6 times as much as the polyester, then because you are using 3 times as much polyester (assuming the same weight / sq ft of cloth and CSM) , it will effectively only cost twice as much - and be infinitely stronger, and last much longer.
No brainer?

On the same theme, I have been monitoring the repair progress of a 24' plywood glass bottom boat ashore here (in Barbados) for repairs. It was built with (allegedly) marine ply on timber frames, and sheathed with a couple of layers of CSM and polyester resin from new.
This fibreglass layer has lifted in various areas; the owner has cut it away and sanded it back to where there is relatively good fibreglass (ie it is still sort of sticking to the plywood underneath).
He then filled (with epoxy putty) various holes in the ply and in way of the joints where you could see daylight through the hull, and applied a coat of epoxy resin to all the bare plywood areas.
I had suggested that he then apply a layer of woven cloth with epoxy - but instead he has sanded the painted on epoxy to provide a key and then applied a couple of layers of CSM with polyester resin.
Not ideal I know, but I am wondering which is the lesser of 2 evils - applying CSM and polyester to bare plywood primed with polyester resin, or to bare plywood primed with epoxy resin, which has then been well 'scuffed up'?
 
There 'might' be an alternative, however, I would like to know just what it is you doing before further advice.

For instance if this it be a rudder or part of a keel, forget the standard resins, you will be wasting time and money and creating another problem that will need more time and more money to rectify.

More information please.

Andavagoodweekend......:)
 
Another vote fopr epoxy I'm afraid,

The previous owner of my boat had sheathed the deck with poly and csm, which wasn't stuck anywhere. I just cut it into 0.5m squares with an angle grinder and lifted it off. in a lot of places water had got underneath and caused untold damage.

moral of story: do it properly or do it twice.

best of luck :)
 
I have built GRP fishing boats to WFA standard, and all non-marine ply, BS 1455 WBP, had to be sheathed. There are boats with such work going back to 1979, it has not come unstuck. My current boat has the ply wheelhouse glassed to the ply deck, since 1995, in rough use, still fine. All polyester. (One point, on one, 1983, boat, the only ply to delaminate on the whole job was marine ply)

First heavily abrade the ply surface, criss cross cuts with a circular saw or angle grinder disc edge, several mm deep.

You want to get resin to attach to the ply. If you just lay on mat/resin, the resin will migrate into the mat from the ply. So brush the ply with accelerator, let it soak in, then resin, which will start to go off fairly rapidly, apply the mat/resin before it does. Remember you need another layer of resin for the first layer of mat. You should, if the work is awkward shapes or inverted, wet out the mat with brush and roller on a scrap board, up to 3 x 600gm at once, then bring it to the work and roll/brush it into the corners etc. If you use more than 3 x 600 there will be too much heat, the resin will turn purple, and the job will not be as strong.

I'm sure I'll be shot down in flames in a moment, but proof of the pudding...I don't expect you will give the job the work that mine gets. I currently have a ply/grp ramp which I knocked up temporarily three years ago for the lobster pots to shoot out over. Bits of it have broken, or worn through but none of the GRP has come off the ply.

Edit: NB: This is above water work. If the ply is immersed, ie rudder, engine beds, bulkheads, in the bilge, it should be totally encased and all fastening holes properly sealed. As with any GRP.
 
Last edited:
If you attack the ply to the extent that is described above, you will get a mechanical 'lock on' ie the resin in all the cuts and scores stuck to the sheet of glass will hold the grp in place like a giant jigsaw piece. privided you prevent water getting behind this coating it should stay there as described, but once water gets in it will rot the ply from behing, and the glass will start to lift, and the water to track under the glass spreading the mischife.

that said, much the same applys to epoxy, it will just spread slower....
 
Look at this. The hauler cabinet is 18mm ply and GRP, totally sheathed, and is only glassed to the hull and deck. The steel davit is only fixed to the cabinet. That has been hauling pots for different people since I built it in 1983. Undoubtedly as people have altered the equipment and drilled new holes there will probably be damp patches in the structure, but I bet it's still there in another 27 years.

jokerhauler.jpg
 
Well there certainly are a variety of ideas on this topic. I am also on three other forums and have asked the same question.

The general concensus seems to be it is OK if you do the job properly and I have been given several methods that have worked for people from keen amateurs restorers to professional boatbuilders who have been doing this since the early 1970's.

What I have proven to myself (not that I was in any doubt) is that there is a great deal of knowledge out there on these forums not only in depth but also in breadth, and I am very grateful for you to take the time to offer your thoughts.

I will now now read and digest all the information and decide how to proceed.

If there any more ideas please feel free to add them to the topic, not just for me but for all the others reading this topic.
 
More pudding proof, I bump into a couple from time to time who have a glass polyester sheathed hard chine cruising dingy. He designed it in the 70s and would have used epoxy if it was then generally available. In the interval of time till now, they have had no probs with the sheathing. I imagine that in taped joints like the Mirror, it gets in at the edge. Less chance with complete coverage.
A
 
if its just sheathing, then epoxy is the way to go. Strip canoes are done this way with a single cloth laid up in epoxy. Earlier polyester resin canoes tended to fail and were difficult to repair. However, if you are making a structural component with the ply as a core and a significant lay up for the skin, then poklyester should be OK, especially if you key it mechanically onto the ply as suggested.

Rob.
 
The cost of epoxy is just one element of construction. The cost of the wood, the time, any other coatings (decorative) etc. I once painted a grass track racing car with paint that was free. Pink wasn't well received though.


The old adage "Why spoil a ship for a ha'porth of tar" really does apply here.
 
I made the same mistake once when tidying up the laminated tiller on our boat. I sanded down the wood that had suffered water ingress and applied polyester resin that I had left over from a car bodywork repair job. It was a !!!**** disaster. Had to clean and sand it all off and start again with epoxy.
 
This was a question about the possibility of Polyester, epoxy is far and away the best. We still don't know what is the bit of ply is being used for. And that matters.
A
 
I understand (from another forum) that the job is on a rudder. Only one way to do it properly, use Epoxy! Wouldn't want to buy a boat that has bodged jobs done on it, would you??
 
Sheathing

There is no doubt the thicker the fibreglass and resin on the wood the better chance of it staying there.
Epoxy certainly sticks better to the wood. However if you can handle the weight more resin and glass to the point where in fact you have a fibreglass rudder with a wood filler rather than f/g sheathed wood.
I think if were me I would sand the wood down to fairly thin then use that as a male mold plug to build up the f/g to something near 5 mm thick. That should last in polyester or epoxy. If it is the whole hull then just paint the wood. olewill
 
You must log in or register to reply here.

Share this page

Top