Interesting CRP on plywood concept.

[FF]

Found this on the George Buler web site.

<center>Polyester Resin and Plywood</center>
Everybody who has been around boat yards since the 70's is familiar with old home made plywood trimarans (usually but not always) with the fiberglass falling off in big sheets. As a result, polyester resin has got a bad rep as a plywood/cloth coating, and today most people use epoxy which sticks much better. However, epoxy costs MUCH more than polyester! Over the years I've heard various ideas to make polyester stick and I know some work because there are many old old glassed plywood boats out there. I've been told one system is to mix some resin "weak" (but HOW weak?) and paint the wood with that. After a couple days, you then glass the boat. I never tried it. Anyway, John Riding in France sent these comments regarding Polyester resin. It sounds interesting!

"I would like to add a little, namely, that the chemicals present in all wood, can and do inhibit the curing of the polyester resin that actually soaks into the wood. So a resin seems cured, and on you go, and add a bit more glass, then finish it all off, then some years and several thousand twists and shakes later the whole darned skin tries to fall off. The resin never cured IN THE WOOD. I learnt fibreglass from one of Australia's masters, Geoff Baker, who had a factory in Mona Vale, Sydney, and built Illingworth 'Top Hats', and the various Brolgas and Boomerangs from the board of Peter Joubert.
Geoff was adamant; Use cobalt napthanate, the standard 'accelerator' additive, 2% or a little more if the wood was 'green', added to the resin BEFORE adding the catalyst. The CN is explosive when directly in contact with catalyst, so TAKE CARE. Mix it well with the resin before catalyzing. With this accelerated brew, paint a primer coat on all the wood to be glassed. It will go off quickly, but, most importantly, will cure right into the wood, and will never let go. Continue with whatever layup you fancy from here on; the stuff is now well bonded. It will fix those rudders that keep shaking their skin off."

 

[Nomad Willy]

FF,Sounds like thinning out varnish really thin the first coat perhaps mixed w a little raw linseed oil to slow the drying out to allow deeper penetration followed by successive coats w more and more varnish to get the best adhesion. Thats the way I frequently do it but a FG coated boat is really a composite boat that is good in itself but dependent on adhesion. Of course the plywood itself suffers from the same problem but suffers very little due to the very extensive technology and machinery that does the job. I think the most viable alternative to F glassing a plywood boat is to finnish the boat properly without FG and taking care of it thereafter. The only significant weak spot w the plywood boat is weak abrasion resistance.
I'd say do away w the FG altogether. The only way to get a stiffer, stronger and lighter boat is w expensive and exotic methods and materials.
What do ya think FF?


Eric Henning

 

[FF]

FG coated boat is really a composite boat that is good in itself but dependent on adhesion.

I surely do not see it that way.

A true composite boast is engineered so the skins carry most of the load and the core keeps the skins apart , and adds insulation and perhaps noise controll.

The GRP slobbered over a plywood boat is simply an attempt at a "permenant" paint job.

It allows a rapid really cheap, build with no molds or tooling to bother with.

We will be replacing the pilot house top and probably entire cabin top forward next year on out lobster boat LUCY .

It will be as strong (but no where as light or as costly) as doing it with a real GRP composite setup, and should take far less time.

A dozen 2x12 (band sawn into 2x5 for camber ) and a couple of sheets of 3/8 ply (laminated to 3/4) will do the job.

The alternative would be a layup of 3/4 oz mat 24oz roving on a purchased core.

Flipping it and forming it over a ridge to give it the required camber and then laying up the exterior 3/4,24.3/4 ,24 ,3/4 .

With the workboat requirement of an easily removable hatch over the engine, 44x88 we have decided too build the hatch opening with cranks , and of course insect screening.

The hatch mifgt be real composite to reduce its weight , and allow light to come thru.

Thinking back to the early days of boat building I remember AIREX used to come with an accelerator sprayed on its surface , and a request for a hot first coat.

 

[skipperdude]

I have had great results by thinning the poly three parts acetone to one part poly add the hardener paint the wood. Just don't use outdoor plywood. (That green stuff.) The acetone really soaks the poly into the wood . Also great as a get rot. It will harden up the soft wood after it cures. At least enought o glass over it again.

SD

 

[johnhinshaw]

SD - do you heat the plywood area you are applying to or go with whatever temp the wood happens to be?

 

[skipperdude]

Never had to heat it* The reaction will heat it enough. The idea is that the acetone will soak into the wood Taking the poly with it providing a tac coat to which the glass and poly *has something to get a grip on.
*
Don't add any surfacing agent to the thinned poly just hardener allow*at least a*day for it to cure.**
This is almost a must do when glassing over plywood. Also be sure the ply-wood is really clean, even sand it first.*Lightly with like an 80 grit. *No oil or moisture anywhere near it.

SD

 

[Arctic Traveller]

I've got first hand experience with sheets of glass coming off a boat.* While running in the ice, there was a loud ripping sound, but nothing was seen until the other tour boat came along side, and noticed a sheet of glass almost twenty five feet long and four feet high,hanging off the side, exposing the core.* Needless to say, it was time to head for the barn, but it was thirty miles back to the nearest dock, and there were fourty five passengers aboard.* Not a real relaxing trip, but no water ever came in the boat.* That was it for the remainder of the season for that boat, but now it's all repaired and making daily trips.* Still, the other side has never been touched as far as any repairs.* Perhaps FF's post explains what happened, but I always wonder if it will happen on the other side some day?*

.............Arctic Traveller