Polyester on exterior ply

[Manie B]

Polyester resin, woven glass on exterior ply.

Gents,
this has come up before, but i would like to know if you have built a boat before using polyester and woven glass on exterior (PF) plywood AND any recommendations / tips you may have eg. preparation

i know that penetration is bad, but this is for a boat that will spend 90% of its time on a trailer and i refer to
http://www.stevproj.com/PocketYachts.html
many of these boats were built and it seems like if the boat was looked after they lasted long.

At the way prices are going now for everything it will be wise to become cost effective, i dont need a boat that will last to infinity.

Still looking at the Jarcat 5m

Any suggestions?

[lewisboats]

Has it been done ...Yes. Would I recommend it...NO! Nowadays exterior ply isn't what it used to be (Too many voids) and poly isn't really worth the time unless you over coat and over weight the boat. If you want the thing to last any decent length of time (5 or more years) it is worh the extra for decent ply and some epoxy. It will probably cost twiceas much for the hull materials which is really only about 30% of the whole boat to do it right...and have something that will last probably 3 or 4 times as long (depending on maintenance)

Steve

[keith66]

In my experience appying polyester grp sheathing to plywood is not as good as epoxy. However if you are not requiring the boat to last for ever and cost is an overriding consideration the following approach has worked for me.
Sand the surface with an angle grinder with coarse sanding disc try to ensure the scratches are across the grain. Apply a primer coat of polyester resin well catalysed. Allow the primer coat to kick of well before applying csm.
I have found that polyester will in fact penetrate further than west epoxy it just dont stick so good!
If you dont prime first the glass will kick of first and there is a high probability of delamination / voids. This is about the only way i have ever found polyester sheathing to be of any use. one thing you must do is ensure all edges are trapped beneath rubbing bands or beadings or the glass will delaminate before too long. Ultimately the cost saving on a deck is not worth worrying about as an epoxy job consumes less materials anyway.

[Manie B]

Thanks guys
i have worked with epoxy very sucessfully and found it easy as well
somehow i was hoping that someone has found a "miracle" new way of working with polyester in the mean time, but alas i suppose i should stop dreaming
anyway should you guys ever have success let us know thanks

[FAST FRED]

The bond isn't great but that is seldome the problem (source of rot).

The penitrations , windows , deck hardware ect MUST be rebedded at intervals. The lighter the construction the more frequently , from flexing.

Worst offenders were the old Taiwan Tubs that would screw a Teak plank set down thru the single layer of glass mat. Instant , invisible rot.

FF

[northerncat]

go bb ply, the jarcats are pretty cool i have often thought of knocking one up in acouple of weeks
sean

[woodenmfv]

Yes, Manie, to know a way 2make Polyester stick to plywood...would be a great solution!

A really often mentioned problem, and easy to solve...almost all say: "just use epoxy instead"

But my reason, researching on that old problem (trying Polyester and not Epoxy) is just the size of the area to cover; I'm building an all new plydeck 52ft x 16ft replacing the old leaking one, what would need something around 20 gallons of resin plus the glass cloth plus hardener.

And the price of Epoxy in UK (you lucky one's overseas) ...its v-e-r-y expensive here, a lot more than in US (after a while I just stopped searching for some cheap epoxy; even did not find some "industrial use"-epoxy, from which is said it's equal quality, just has'nt that broad-temperature-range - but of course much cheaper than the marine stuff. Seems over here exists a kind of "epoxy-kartell",keeping prices high *g*)

So, I looked through a lot posts about "fibreglass-topic"; one guy posted an interesting link to good-old-backyard-buehler's page, where someone wrote about a way to make that (stinki)Polyester stick to our plywood...
"...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..." http://www.georgebuehler.com/Polyest...20Plywood.html He also describes WHY it normally don't stick; all makes a bit sense to me, but I'm no expert in that field (as you expected of my questions)

Hm, any chemist around here to cope that description above? Maybe that works, what do you folks think?

PS: Hey Manie, that cat looks really cool - easy to carry and for sure fun sailing! .. I think, there's a South-African Company called "Elichem", providing Industrial epoxy to Mining-companies. I stumbled over while my research; they have a little branch over here to - they may have good priced epoxy (...just 'cause I saw you are from S.Afr.)


PS 2: I hope George Buehler excuses me the "old" in "good-old" :wink: I can't imagine my bookshelf without his book "backyard boat building" (though I suppose my ol'bathtub never saw any backyard even from far)

[Meanz Beanz]

Originally Posted by Manie B

this has come up before, but i would like to know if you have built a boat before using polyester and woven glass on exterior (PF) plywood AND any recommendations / tips you may have eg. preparation

I once had an old plywood boat that had been covered in polyester and chop strand from new. She was as good as the day she was built when I owned her. The only thing that I could identify that they had done over and above the application of the GRP was scoring a 50mm diagonal grid on the surface of the ply. It look quite aggressive in the areas that could be seen.

[PAR]

Polyester doesn't stick for a simple reason, it's rate of elastic modulus, compared to the materials it's used in conjunction with on wood. The substrates have higher elongation rates as a rule, so the resin system fails.

The scratching method, mentioned by Beanz is obviously an attempt to address this adhesion issue, other wise they wouldn't bother. In other words, they know they have a set of problems, but don't understand it completely so they're trying sometime to help.

I've seen all sorts of methods attempted, including driving nails or tacks until the heads are just barely showing, staples, you name it, but it doesn't address the basic issue.

The only method that works is to use the plywood as a mold and lay enough resin soaked fabric on, that the plywood no longer needs to be part of the structural element. In the event of massive wood rot, it doesn't matter, the new laminate is strong enough to support the loads. Of course this is the way it's done of work boats and it produces a very heavy laminate, in addition to the wooden elements of the structure.

[Jimbo1490]

There are several reasons why polyester resin doesn't stick all that well and epoxy does. There's the elastic modulus as Par mentioned. There are flexibilizing agents that might be used to alleviate this problem, but like any resin modifier, you gain in one area but lose somewhere else, tensile strength this time. And poly resin does not have a whole lot of tensile strength to begin with, so this is not considered a good trade-off which is why it's not done.

The next problem that plagues poly resin's adhesive properties is that it always shrinks as it cures. All resins shrink some during cure, but poly shrinks A LOT, like 3% or more in some cases. So it actually retreats from close contact with the surface that you are trying to bond to. This is not good. Epoxies typically shrink .1-.3%

The third problem has to do with surface tension. Surface tension in a liquid prevents 'wetting'. Water is wet for instance, but it does have some surface tension. When you put soap in the water, it kills the surface tension and now the water is 'wetter' and adheres to things better, capillaries better, spreads into a thin film, etc. Oils have about *zero* surface tension and wet and cling to surfaces very well.

Well it's the same with poly and epoxy. Epoxy is just 'wetter' than poly and seeks out all the little micro cracks, craters and divots in the surface down to the molecular level which gives you way better adhesion.

And with SO MANY different curing agents and modifiers (and much higher native tensile strength to work with) you can tailor the cured state properties of epoxy resin to match the elongation, flexibility, etc of wood quite easily so the bonds will be so much more permanent.

Don't waste you time with poly!



Jimbo

[ondarvr]

.Adding cobalt will speed up the gel time, but this can also work against you.

Some types of wood do have resin in them that can inhibit the cure of polyesters, water will do the same thing, so the dryer the wood, the less of a problem this is. Extra Cobalt "may" help it cure if that's the issue, but this faster gel time will also give it less time to penetrate into the wood, possibly creating a different issue. Resins can be purchased with gel times from 5 minutes to over an hour, so you need to know if the shorter gel time will be too short for the resin and temperature at the time of use. Plus there are so many different formulations of polyesters and blends of them with other types of resin, and then add in the number variables in the types of wood that may be used, this results in you never knowing just how good the bond will be until after the fact.

The fact that they coated the wood with resin very well before glassing helps a great deal, most people don't do it well enough.

The problem is polyester just won't work as well over wood as epoxy does, whatever reason there is for this, there's just no way to get around it.

[oldsailor7]

Try Vinylester resin. It is better than Polyester and not as expensive as Epoxy.
Having built eight wooden boats, the first three with Glass/Polyester and the remainder with Epoxy, --using the Wood Epoxy Saturation Technique. (Not WEST system epoxy----too toxic !!).

I would never build a wooden boat again using glass/polyester sheathing.

[sproxxin]

I just got the price for epoxy from harveys and it works out to about R 145.00 per kg. The price of UPE resin is about R 42.00 a kg. I'm planning on building a stitch and glue canoe. Maybe it would be cheaper to build a fiberglass canoe!

[Manie B]

Hi Sproxxin

welcome to BD.net

there is a TON of info here on BD.net

you have alot of reading to do
this a an old thread and we have progressed a lot over the past couple of months

by the way where are you
what boat are you thinking of building

i am a huge advocate of stich and glue = marine plywood and epoxy
i can assure you that your canoo can be a smart boat in epoxy

i am of the opnion that a ply/epoxy build is the same cost ultimately as polyester and glass

[FAST FRED]

Contemplate Dynell , instead if fiberglass, doesn't cost much more , and is more flexible.

FF