Osmosis with epoxy.

Hi,

Is there a risk of osmosis with epoxy resin?

In a case of fiberglassing and epoxing a whole (in and out) canoe hull made of wood (waterproof ply) will osmosis be a concern?

Thx in advance.

Peace.

 

First just a little explanation... No barrier coating I have ever heard of is completely impervious to water intrusion. The goal with them is to minimize to rate of water absorption to a level than is non destructive, and controllable. That being said epoxys are the best material currently available at minimizing water intrusion, but there are limits.

For instance a lot of water intrusion into hulls actually isn't the fault of the barrier coat or the construction resin, but is instead caused by glass fibers that penetrate through the resin layer. This is particularly problematic when using chopped strand matt, or a chopper gun. These small fibers when they poke through act like a straw for oscmatic pressure and give a rout of entry for water. When this happens I doubt (though I have no evidence either way) that the specific resin formulation has any effect on water intrusion.

This is why after the hull is finished almost all boats have an exterior barrier coat of epoxy applied. Of course some materials like wood are often completely encapsulated before the construction process, then again coated with a barrier coat after construction.

 

Just take care you make it properly so no worries..

BR Teddy

PS Stumble gave allready some good advice.. don't ever use CSM with epoxy, only fabric

 

Thank you for your answers.

Therefore, is it wise to think that meticulously glassing the whole hull is same life span as an outside glassed hull with the inside of the hull leaved bare/stained (for evaporation)?

P.S: I am sitting front of my ply hull not daring to glass it...

Peace

 

Epoxy is good , it doesn't hydrolise when it's saturated, it also saturates at a much lower moisture content.
The esters absorb more moisture and can and do de-esterise due to the water content, it's exacerbated by a host of factors some of the esters are a lot better than others.

In your situation anything will work since the vessel will be out of the water most of the time, trailerable boats stored on the trailer between uses for example don't blister the way you are worrying about..

 

The glue in plywood is as much of a barrier to water penetration as epoxy is so leaving the inside bare is not going to allow evaporation of water from anything other than the interior surface layer. If you use epoxy to seal it then the chances of water getting into the surface ply are greatly minimized and you don't need to let evaporate. WEST system is Wood Encapsulation which means every piece gets coated multiple times with epoxy creating a very tough barrier to absolutely minimize water ingress to the ply to start with.

 

I use a really low viscosity 2 pack epoxy that soaks in before I put anything over the wood. Then I glass over that, seems to work well. A lot of folks used to just paint polyester resin over the exposed wood and I had an old plywood sailing dinghy which was treated like that inside and glassed outside about 20 years ago and it's still in good shape although it was a good quality ply.

 

A resin primer

Here is a little primer on resins.

Polyester is the worst for osmosis and water intrusion because it is mixed with the monomer styrene, but not all the styrene is used in the cross-linking chemical reaction that turns the resin into a hard plastic. Some is left over, and it evaporates, and that is what causes the "fiberglassy smell" for fiberglass. When the styrene evaporates, it leaves tiny molecular holes in the laminate through which water can pass. This leads to osmosis.

The next grade up, vinylesters, also have styrene monomer in them, but much more of it is used in the curing chemical reaction. A lot of the styrene cross-links with itself, creating polystyrene in the laminate. Very little styrene flashes off and leaving holes in the laminate. The resin is very nearly, but not 100%, solid plastic. Chemical tanks and underground gasoline tanks are made of vinylester resin because the vast majority of liquids, and environmentally dangerous liquids, cannot penetrate vinylester.

The top grade is epoxies. They are made of two chemicals which, when catalyzed and cured, create a 100% solid plastic. There are no holes, so there is no way for liquids of any sort to penetrate. This is why epoxy barrier coats are used on boat bottoms, and why epoxy-built boats withstand being in the water without trouble.

Generally, in my laminates, I do specify some extra layers of chopped strand mat in the outside layers because if water were to get into the laminate and travel along the fibers, the fibers are usually only 2" to 3" long (50 to 75 mm) and the water usually travel only to the end of the fiber, not very far. If you were to put glass cloth or knitted fabrics directly behind the gelcoat or on the very outside, with no mat as an additional barrier, then it can happen that if water gets into the laminate, it can travel a great distance along a poorly wet out fiber to another part of the boat. So this is why is it always good to have a thickesh layer(s) of mat on the very outside of the laminate, between the gelcoat and the base laminate.

That's my 2 cents. I hope it helps.

Eric

 

Thank you very much for these detailed explanations, it helps.

I will pre-coat then fiberglass my whole hull with my epoxy (and fabric).

Thanks again

Peace

 

You are worrying for nothing. I have never heard of a canoe getting osmosis.

It only happens to boats that spend a lot of time on the water, or with water in them.

 

epoxy is essentially waterproof which is why fiberglass hulls (built with porous polyester resin) often apply a barrier coat of epoxy.

Your problem is not the porosity but getting a good bond to the wood with the epoxy. Wood expands and contracts with moisture, epoxy with temperature.

The fix is several coats of solvent thinned epoxy which will bond better to the wood due to the tiny amount of penetration due to the solvent. Also the solvents will add some flex to the epoxy (for a little while).

 

That's false and faulty all together... :!:

 

Yes Pauloman, thinning epoxy has been discussed at great length and great detail in these forums, and as Teddy says, is completely the wrong idea.

If you dont feel like doing the searching, I am happy to post links to all the relevant info.

 

I stand by my comments. I have been employed in the epoxy industry (an own an epoxy company) for 20 years and deal with epoxies and epoxy users on a daily basis. I also currently own 30 small boats - kayaks rowboats etc. Many I built or restored.

 

That explains a bit of your obsolete methods, but not a good excuse.. About a good time to revive some basics me thinks..
BR Teddy