Sealing Plywood before Applying Epoxy

[minaret]

Originally Posted by Delancey

Minaret,

I am interested but not entirely surprised to hear you say this. I want to say I have read at least one epoxy resin manufacturer advise specifically against thinning their product with any kind of solvent. In your experience this is fine? Did you ever do adhesion tests to compare thinned vs full strength?

Cheers

Seems like we've discussed this several times before. You were there for at least one of them.




http://www.cruisersforum.com/forums/...ot-113845.html

[Delancey]

I'm not trying to be snarky, I'm just interested. Yes, I was there for that one but you didn't say in that thread that you thinned. Frankly, I place a lot of stake in the things you say.

A manufacture may make certain recommendations, that doesn't mean your practical experience doesn't suggest otherwise. I have never thinned and tested the results so I really have nothing to go by, hence the question.

[minaret]

Originally Posted by Delancey

I'm not trying to be snarky, I'm just interested. Yes, I was there for that one but you didn't say in that thread that you thinned. Frankly, I place a lot of stake in the things you say.

A manufacture may make certain recommendations, that doesn't mean your practical experience doesn't suggest otherwise. I have never thinned and tested the results so I really have nothing to go by, hence the question.

Me neither. Perhaps I'm confusing my threads, there have been many on this subject. Suffice it to say that I believe when WEST says you should not thin their resin, it is just CYA. And with good reason. They just do not want the amateur user getting in over their heads. Viscosity matters when it comes to penetration, and penetration matters when it comes to dimensional stability. This is why when I say reduce for a sealer or split coat, I give numbers and a product. I've seen WEST mixed this way penetrate all the way through 1/2" ply and come dripping out the other side. It's not necessarily about promoting adhesion, but about stabilizing the wood to prevent problems later. And adhesion.

[HappyMdRSailor]

Hooray!

Let me take my "oh boy" back then....

[mpatter894]

don't put any sealer on it just coat it with resin lay the cloth on coat the cloth with a brush and let it set, simple as that the first coat will keep the cloth from moving so much the type of resin is up to you after that is done put another coat on with a foam roller and tip it off with a dry brush to get a nice smooth finish

[Delancey]

I've never had the occasion to need to thin epoxy but that doesn't mean I can't imagine situations where it might not be a bad idea.

I am no chemical engineer but as I understand it, one of the things that epoxy has going for it as a coating for things like tank lining and encapsulating wood is that unlike polyester resins, most epoxy resin systems for laminating and encapsulating don't have diluents that off-gas and that this was also why generally speaking epoxies had very low vapor pressure and didn't stink the place up quite like poly.

I also could have this part wrong but I thought the microporosity resultant from the off-gasing of solvents when poly cured left behind voids that are the pathway for water intrusion in the development of blisters, and that because epoxies lack this evaporative component they have high chemical resistance.

Like I said, I never had occasion to thin epoxy but it would seem that as a first coat on wood it could make sense and I would expect you are coming back with your second coat within the window.

That first coat isn't doing anything as far as encapsulation goes, what it is doing is preventing subsequent joinery or lamination from becoming "glue starved" as the surrounding wood or fiber absorbs epoxy out of the joint.

As long as whatever solvent you use as a diluent doesn't alter the chemistry of the epoxy cure it should be fine. I would think acetone as the one I would reach for first but if MEK than so be it.

Could be fun to soak a piece of MDF or particle board with it and then do an immersion test to see what happens.

[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.

[minaret]

Originally Posted by Delancey

I've never had the occasion to need to thin epoxy but that doesn't mean I can't imagine situations where it might not be a bad idea.

I am no chemical engineer but as I understand it, one of the things that epoxy has going for it as a coating for things like tank lining and encapsulating wood is that unlike polyester resins, most epoxy resin systems for laminating and encapsulating don't have diluents that off-gas and that this was also why generally speaking epoxies had very low vapor pressure and didn't stink the place up quite like poly.

I also could have this part wrong but I thought the microporosity resultant from the off-gasing of solvents when poly cured left behind voids that are the pathway for water intrusion in the development of blisters, and that because epoxies lack this evaporative component they have high chemical resistance.

Like I said, I never had occasion to thin epoxy but it would seem that as a first coat on wood it could make sense and I would expect you are coming back with your second coat within the window.

That first coat isn't doing anything as far as encapsulation goes, what it is doing is preventing subsequent joinery or lamination from becoming "glue starved" as the surrounding wood or fiber absorbs epoxy out of the joint.

As long as whatever solvent you use as a diluent doesn't alter the chemistry of the epoxy cure it should be fine. I would think acetone as the one I would reach for first but if MEK than so be it.

Could be fun to soak a piece of MDF or particle board with it and then do an immersion test to see what happens.

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.

[Terra Nova]

Originally Posted by Chenega

There is no need to thin epoxy and it will reduce the adhesive strength. Epoxy works because of a chemical balance - change it by adding another chemical (thinner) and you change the bonding, curing, and strength characteristics of the product. This is the base coat of your project and you don't want to see the next layers fail.
I often do a "hot coat" by warming the epoxy lowering the viscosity and while still "green" finish up the glass work. Don't heat the epoxy, but warm it to 80 - 85 degrees as well as the plywood. Remember, this will shorten the pot life and working time.
Here's some info - typical with most epoxies.
Good luck

WEST SYSTEM - Projects - Thinning WEST SYSTEM epoxy

Good post

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.

Bad dog!

Use the right epoxy for the job (like WEST) and NEVER put anything in it which has not been recommended by the manufacturer. ESPECIALLY do NOT add solvents.

[minaret]

Originally Posted by Terra Nova

Good post



Bad dog!

Use the right epoxy for the job (like WEST) and NEVER put anything in it which has not been recommended by the manufacturer. ESPECIALLY do NOT add solvents.

Why not? It's not like WEST explicitly says not to. Just that you need to be aware of what you're doing. Can you refute my points on the subject more precisely, with links?



Many have linked the above WEST article (from 1999!) before. Read it carefully and you'll find they agree with everything I'm saying.


When we built Cascadia (77' Schumacher ULDB), it was all cold mold/carbon hybrid in WEST Proset. Vast sums were spent on chemical engineering and WEST reps came out often. When we vacuum bagged the cold molded hull panels, they got sealer coats first.


But I'm just a lowly boat building dog, why listen to me? Woof!

[Terra Nova]

Min--in nearly 40 years of boat building I have used barrels of WEST epoxy. And I say, without hesitation, that adding 25% MEK (as you suggested) will significantly reduce the mechanical properties of the epoxy. And I will add that there is absolutely no reason to even have to thin the proper choice of epoxy. And repeat: Never, ever, add solvents that the manufacturer has not specifically recommended.

Experiment, if you wish. As for links, read the Gougeon brothers book on boat construction.
https://www.maineboats.com/print/iss...t-construction

[minaret]

Originally Posted by Terra Nova

Min--in nearly 40 years of boat building I have used barrels of WEST epoxy. And I say, without hesitation, that adding 25% MEK (as you suggested) will significantly reduce the mechanical properties of the epoxy. And I will add that there is absolutely no reason to even have to thin the proper choice of epoxy. And repeat: Never, ever, add solvents that the manufacturer has not specifically recommended.

Experiment, if you wish. As for links, read the Gougeon brothers book on boat construction.
https://www.maineboats.com/print/iss...t-construction

Pretty poor refutation of my carefully laid out points. Yes, it will reduce the mechanical properties. But it's a sealer coat, you aren't laminating with it, only stabilizing the substrate to prevent joints from moving, warpage, expansion and contraction which is dissimilar to the glass skin, etc. So a loss of strength is irrelevant. The reason for thinning was already laid out, to promote penetration and better stabilization. It's called the "wood epoxy saturation technique" for a reason. If you are working with ply too thick to saturate neat but planning to glass to it, saturating by thinning is the way to go. And in this case, the manufacturer has recommended this thinner for this use, to me, in writing and in person. Just passing it along. It's a no brainer really, glass to dry unstable kiln dried ply, or glass to the same piece of ply saturated in epoxy and stabilized. Choose to not believe if you like, I couldn't care less if your glass on ply delaminates.

[Richard5]

For anyone using MEK please read the Material Safety Data Sheet (MSDS).

http://www.google.com/url?sa=t&rct=j...NDJpk_auFkQW4g

Fire depts. hate MEK because it generates intense heat and creates it's own oxygen as it burns. MEK can be extremely flammable under certain conditions which, it turns out, those same conditions are usually present for the 'backyard builder'.

[minaret]

Originally Posted by Richard5

For anyone using MEK please read the Material Safety Data Sheet (MSDS).

http://www.google.com/url?sa=t&rct=j...NDJpk_auFkQW4g

Fire depts. hate MEK because it generates intense heat and creates it's own oxygen as it burns. MEK can be extremely flammable under certain conditions which, it turns out, those same conditions are usually present for the 'backyard builder'.

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...

[Dale Hedtke]

I'm in agreement that using any kind of thinner with epoxy is poor practice.
The idea of warming the epoxy to reduce its viscosity is a good one, especially when working in a cool work space.
HOWEVER, be careful with heating the wood itself. Here's why: if the wood is heating up (temperature rising) while the epoxy is curing, gases trapped inside the wood can expand and blow bubbles in the curing epoxy.
Another suggestion is to use a low viscocity resin to start with. I haven't used WEST in a long time, but it used to be quite thick. Look at the characteristics of the MAS product line or their copiers.