Sealing Plywood before Applying Epoxy

[Cormorant]

FWIW, Robb White, the legendary Florida boatbuilder and writer, used to heat up his whole shop, boat included, to as hot as it could get -- maybe 100 degrees, and let the boat come fully up to temp overnight. Then he would apply a coat of slow-cure epoxy and immediately turn on the AC and drop them temp as fast as possible. He felt that the cooling of all the interstitial air spaces in the wood would draw epoxy into the wood on a microscopic level.

[s/v Jedi]

So, let me add my method, which is, as always, based on the scientific approach (study options, test etc.).

I take a piece of plywood big enough for all the small pieces needed later. If it's a lot, I may use half sheets (4' x 4') to make handling easier. I then start by prepping the plywood, a quick sand and vacuum it, then fill any damage or imperfections.

After that, I heat the plywood with the sun or a heat gun. I then drop fiberglass cloth over the dry plywood and pour mixed (slow) resin in the center. Use a spreader to distribute it and work out any air.

When the epoxy starts gelling, I do the next layer with the spreader. If the surface is going to be a finished surface, I will then switch to layers using a roller (pro's can do almost everything with the spreader) untill the weave is filled up and then some extra to provideroom for sanding smooth. If it isn't going to be a finished surface, I stop after the second layer with the spreader.

Testing:

Adhesion is much stronger than the veneers. When you pull the cloth with vice grips, a ply of veneer comes off with it. With top quality plywood, like Bruynzeel, the plies are also bonded stronger than the wood, so that half a veneer ply comes off with the other half still bonded to the next ply underneath.

Impact resistance is very good. A good smack with a hammer shows a milky white impact zone, but at that stage the surface is still intact and smooth.

This is not a method I devised but rather one used by many professionals. I added the heating after having problems with tiny bubbles which are cause by air inside the plywood that expands with the exothermic reaction of the epoxy. By heating, you expand it much moreso that it contracts later on, sucking epoxy resin back into the pores with it.

I do not like pre-coating because it makes the wood fibers stick up which messes with the cloth being positioned.

Here is a picture of my better half pouring a first batch of epoxy:

[s/v Jedi]

And here is the result after the first layer has been applied: 100% transparancy shows complete wet out of underlying plywood.

[s/v Jedi]

And to show finished result: this is a liner made from the ply shown in the pictures above after two layers of Awlgrip 545 plus two layers of topcoat, all brushed.


We're now 9 yearsof full time use later and the surface is as beautiful and smooth as in the picture, even after dropping in vodka bottles on a regular basis

[Delancey]

Originally Posted by minaret

Micro porosity is a good thing in a sealer coat, it promotes adhesion of the next coat which soaks into the pores. The other side effect of reducing epoxy, a slightly more flexible cure, is also a good thing in a sealer coat. A dimensionally stabilized substrate is a big deal, if you must glass to wood. Yes, reducing does alter the chemistry. But in a known fashion which can be used to your advantage. I prefer MEK to acetone because acetone is too volatile, you will need to stop and re reduce in the middle of coating. I suggested a slow hardener because this reduces the amount of pin holing from off gassing dramatically, as the resin is still wet by the time all solvents have gassed out. If you do this in high temps with a fast hardener, you will indeed see severe pin holing. If you do it in normal working temps with a slow hardener (which also allows more time to soak in), it works great. The slow evaporation rate of MEK also helps keep the epoxy watery thin for long enough for it to penetrate deeply. Acetone gasses off the moment you put it on the surface.

Thanks for sharing your practical insight. I tend to believe the CYA theory as being the most plausible explanation as to why a manufacturer would not recommend thinning besides the fact that thinning can result in greater coverage depending on the application which means people buy less product.

I think it's interesting that some who would condemn this practice seem to be unable to offer a solid reason why. Obviously, you wouldn't want to thin epoxy for most of the applications you would want to use epoxy for, but that's not what Minaret is saying is it? He's not saying thin epoxy you're using for encapsulation is he?

A fact of the matter is that there are any number of epoxy-resin base paint systems that most certainly do use a variety of solvents as diluents and reducers so it's not like epoxy resins are fundamentally incompatible with solvents in some way.

I think it goes without saying people have to decide for themselves what works for their needs, I myself won't be afraid to do a little experimenting when the opportunity next presents itself.

I mean seriously, don't trust what people say on the internet, prove you are right. It's called scrap material for a reason so get busy and do some tests. Then when you are done come back and report the results to the internets.

In the meantime I'll take Minaret's word on the matter.

[minaret]

Originally Posted by Cormorant

FWIW, Robb White, the legendary Florida boatbuilder and writer, used to heat up his whole shop, boat included, to as hot as it could get -- maybe 100 degrees, and let the boat come fully up to temp overnight. Then he would apply a coat of slow-cure epoxy and immediately turn on the AC and drop them temp as fast as possible. He felt that the cooling of all the interstitial air spaces in the wood would draw epoxy into the wood on a microscopic level.

Couldn't agree more with this practice, and you will note in the other thread about this which I linked to earlier, I was the only person who mentioned this concept. It makes a huge difference. And in fact, I have seen a half million dollar part sawn up and thrown in the dumpster because they refused to listen to me and did just the opposite.

[SVTatia]

Originally Posted by AD 2

Thanks everyone! I've decided to skip the dubious merits of applying the 'Old Salem' sealer and go straight to slapping on an initial coat of epoxy resin, the same type I'm using to glass, no thinner but including heat. Hopefully the window will be nice and wide tomorrow to start glassing. Will let you know how it goes.

AD 2, you may want to protect the ply that is not being laminated, I mean, the edges and the other side, but specially the edges. For that you use the same epoxy and apply two coats with a brush.

To seal wood and plywood before painting or varnishing, I use (System Three) S1 Sealer. Its a very thin epoxy, like water, its quick and easy to mix (1-1) and costs much less than the regular laminating epoxy.

[minaret]

Originally Posted by SVTatia

AD 2, you may want to protect the ply that is not being laminated, I mean, the edges and the other side, but specially the edges. For that you use the same epoxy and apply two coats with a brush.

To seal wood and plywood before painting or varnishing, I use (System Three) S1 Sealer. Its a very thin epoxy, like water, its quick and easy to mix (1-1) and costs much less than the regular laminating epoxy.

S-1 sealer is 25-40% xylene, 10-30% MIK. Lot of thinner, isn't it? Makes my 15% MEK seem pretty tame....

The S3 plant happens to be right next door for me....


http://www.systemthree.com/reslibrar...ealer_MSDS.pdf

[sailr69]

Originally Posted by BriaF

M E K

Methyl ethyl keytone

A thinner
Very aggressive
Carcinogen has been known to cause
Cancer.
Will melt paints rubber plastic + more.
Epoxy will soften even after curing
Some times used to thin epoxy , I suspect
that will reduce bonding strength
Good for cleaning tools & brushes after use
with epoxy.

MEKP is used as the catalyst for polyester resin as well, for thinning epoxy I would use alcohol,

[sailr69]

Originally Posted by Cormorant

FWIW, Robb White, the legendary Florida boatbuilder and writer, used to heat up his whole shop, boat included, to as hot as it could get -- maybe 100 degrees, and let the boat come fully up to temp overnight. Then he would apply a coat of slow-cure epoxy and immediately turn on the AC and drop them temp as fast as possible. He felt that the cooling of all the interstitial air spaces in the wood would draw epoxy into the wood on a microscopic level.

what ?? he should learn to vacuum bag, a lot cheaper as well

[minaret]

Originally Posted by sailr69

what ?? he should learn to vacuum bag, a lot cheaper as well

The failed half million dollar part I mentioned was bagged. Even a vacuum pump cannot overcome expansion of air trapped in the substrate due to dramatic rise in temp during the bagging process. In this case it was balsa core stored in an exterior shipping container overnight during sub freezing temps, and then taken in to a warm shop and bagged in the morning. I told them this was a recipe for disaster, but they just couldn't see heating the core overnight before bagging.

[sailr69]

Originally Posted by minaret

Yep, it's the good stuff! Doesn't make me anywhere near as nervous as those fifty gallon drums of acetone we keep laying about though...

I have to laugh at this because I used to use large drums of acetone at RUPP industries in our design center for fiberglassing and then the VP of engineering told me to get something else because of the fire danger. SO I ended up with using MEK (much safer they said) the MEK would burn our hands melt the resin out of our FRP parts. everything near the fiberglass shop got sticky. everyone complained. seemed to take forever to get rid of that drum. the next drum was acetone. we bought a bigger fire extinguisher! lol

[minaret]

Originally Posted by sailr69

I have to laugh at this because I used to use large drums of acetone at RUPP industries in our design center for fiberglassing and then the VP of engineering told me to get something else because of the fire danger. SO I ended up with using MEK (much safer they said) the MEK would burn our hands melt the resin out of our FRP parts. everything near the fiberglass shop got sticky. everyone complained. seemed to take forever to get rid of that drum. the next drum was acetone. we bought a bigger fire extinguisher! lol

Yea, definitely not good for a regular use solvent, special purpose only. It's bad nasty.


Back when I was doing factory floor work, I got to run a big industrial acetone still. Ever see one? Pour in all your dirty 'tone, and the machine boils it to evaporate and condense, giving you clean 'tone and a big cake of toxic sludge. Fire departments just love those...

[Terra Nova]

Originally Posted by minaret

...Use the epoxy you will laminate with heavily reduced. I like 25% MEK or even more...

Very bad tech.

From Gougeon brothers, themselves:
"Adding solvent is a quick, simple method of thinning epoxy, but unlike using heat to thin it, the strength and moisture resistance of the cured epoxy are drastically affected."

From:
Thinning West System Epoxy

[Cheechako]

Originally Posted by minaret

I always seal before glassing. Use the epoxy you will laminate with heavily reduced. I like 25% MEK or even more, but be sure you are using a slow hardener and catch the chemical bond window. Never seal with anything but the resin you will be laminating with.

Yep, anytime you glass to wood put on a bonding coat first so when you laminate the resin isnt absorbed too much into the wood.