Thread: Epoxy coating,sealing, sheathing gaboon/okoumy plywood

These surfacing veils are really mat, in this case Kevlar. Mat sucks up resin like a sponge, so weight savings will be marginal at best. A single epoxy coat doesn't really seal things, film thickness does. This is why you hear 3 coats as the recommendation on plywood, which will produce a 10 mil thick coating and this is considered waterproof. A single coat will be less then 3 mil thick which isn't going to do much.

The second and more important consideration is abrasion protection. In this vain there are other materials tough most are also resin suckers too, which is why you see light cloth as the recommendation.

Thanks Par,
I realise veil/tissue is resin rich, thats kind of the point, would it be an alternative to 3 resin coats with the added benefit of some reinforcement without adding fabric ?

From a structural and abrasion stand point you wouldn't gain much, it's just not very dense. These veils are intended to cut down print through and off some decortive finish in some cases (printed veils). If you want to keep weight down, apply two neat coats and while the second is tacky, lay down 2 ounce cloth and wet this out. This will offer sufficient film thickness and imbed the cloth at the same time. It takes very little goo to wet out 2 ounce (68 GSM) cloth and it'll offer about as much abrasion resistance as other light cloths.

From a structural and abrasion stand point you wouldn't gain much, it's just not very dense. These veils are intended to cut down print through and off some decorative finish in some cases (printed veils). If you want to keep weight down, apply two neat coats and while the second is tacky, lay down 2 ounce cloth and wet this out. This will offer sufficient film thickness and imbed the cloth at the same time. It takes very little goo to wet out 2 ounce (68 GSM) cloth and it'll offer about as much abrasion resistance as other light cloths. Simply put, a mat or veil will use more resin than a very light cloth, so no real weight savings. Lastly, Fibreglast has some cool stuff (ChromeVeil), but it's about as costly as it gets, compared to other similar products.

As well as film thickness there is another point to the three coats..and it applies to other things as well as epoxy.

if you apply some sort of resin or paint to a porpous surface...

the first coat will be partly absorbed by the porosity and texture..but it will leave some voids incompletely filled and some protrusions poorly covered......this coat will set up ( partly or competely) and seal up the timber...the majority of absorbtion will cease.

The second coat will fill the remaining voids and finish any incomplete sealing.

The third coat provides a continuous film

Even with some sort of mat applied to a surface, you realy need 3 coats...the above applies when mat is used too.

If mat is applied and there is insufficinet resin involved, either the mat or the timber below will not be properly saturated.

If you want to save weight.....be particular about the evenness and application rate of all 3 coats of epoxy.......use only sufficient epoxy to do the job..it is all to easy to apply more epoxy than necessary.

cheers

Soundman has it absolutely correct and I'll add if the previous epoxy coats are sanded to level things out, you'll be knocking off some highs, potentially (likely) exposing some raw wood below, so the subsequent coats help seal these back down and provide enough film thickness to make it waterproof.

I'm not a fan of mat products, mostly because they add no strength, very little modulus and only a little hardness (only because of the resin content and if thick enough). In this case, a light cloth would offer better abrasion resistance, use less resin to wetout and likely less weight too, if it was well applied.

Hell man you better go back to the text books mate.

You could start with The gudgeon brothers on boat building and Warram on composites

The whole point of various mat products is to increase strength..and considerably so.

Maybe you don't like the smoothing products...but to say that "mat products" do not increase strength is simply rediculous.

cheers

Well, seeing as I'm one of the consultants for those in the industry and for the edited versions of the "books", maybe it's you that needs to perform the studying. The physical data for how mat performs in a laminate is well documented and one of the reasons the industry has progressively looked for alternatives in the last 3 decades. Most production boats have very little mat in them now, substituted with kitted and woven fabrics, which is is direct comparison to the all mat laminates that were the rage in the 50's through the 70's. Simply put, mat is heavy (resin content) and way too flexible for it's modulus (elasticity), compared to other materials and laminates. Sure you can use mat, but if resin use, weight and cost are considerations, which is alsways the case in performance applications, you'll not see any significant amount of mat employed, except as surfacing agent for abrasion (better off with a polyacrylic or polyester) and to prevent print through, which requires very little mat, typically 2 ounce (68 gsm) or less (preferably).

I don't know you experence level, though am famular with your argumentive nature. I'm reasonably confidant folks here and elsewhere will attest to mine, so maybe you can provide some data to support you claim that mat increases modulus in a laminate, particularly compared to the other usual elements found, such as woven or knitted fabrics (not all of which are 'glass BTW). Again the data is widely available, so look up resin content, strength and stiffness figures for hand laid mat, compared to cloth, uni and biax, etc. The first thing you'll see is it's all but imposable, to get a reasonable resin/fiber ratio, which directly affects performance in the matrix and one reason mat is so, well, weak. The other thing, not as widely published (though still available) is the discontinuous fiber aspect. Loads can't transmit through the laminate along fibers axially, so they have to jump across gaps, to the nearest fiber, which just leads to fractures and delamination. This is why uni and biax are so much stronger than cloth and much more so than mat. Continuous fibers can transmit load paths much easier, before you reach the elastic limit of the resin or fiber. This is the key to understanding how laminates work and are designed. But hey, what do I know.

don't lose sight of the fact that my proposal was to use Kevlar veil, where the resin rich property is desirable, further researching on my part also reveals that some manufactures offer continuous fibre veils which may help the physical properties in some small part, especially given that i want to use it on both sides of the ply in way of 3 coats of resin. These products are recommended for tank linings in aggressive environments so
I'm still not convinced that it can't replace resin alone, on the exterior it will receive at least a light coat of epoxy Hi Build.

I appreciate your participation in the discussion.

Kevlar's real benefit is tensile strength and penetration resistance, which means its best location in a laminate is on the inside face, as far from the impact point as practical. By it's mature it is a resin sucking material and not easy to work with, but it can be done in the back yard. In high solvent resistance environments, you'll want the veil on the outside of the laminate, for obvious reasons. If looking for abrasion, I'd recommend a Xynol sheath, which is also resin rich, but a lot cheaper than Kevlar and easy to work with. Epoxy has very good chemical resistance, so the resin rich fabrics make sense in some applications. Given your desires, I'd say it's a crap shoot, without some coupon testing (little samples of each), so you can decide if the light weight and strength of a very light Kevlar veil offers as much as say Xynol. The Xynol will be heavier, but will this weight gain be enough to tip the scales, considering cost and applications issues.