adhesive or epoxy for T&G assembly

[missinginaction]

Rather than reuse the horribly designed fiberglass flybridge on my 1973 Silverton restoration, I decided to design and construct my own.

I've got all the plywood panels and cleats/beams prefabbed and ready to go.

This is a tongue and grove assembly that nicely interlocks for a sturdy design.

I have the option of bonding everything together with epoxy alone, or using 3M 5200 or Sikaflex in the dado joints and then finishing with an epoxy fillet where the joints are visible.

I'm wondering if any of you have any input on the best way to proceed. I'll post a couple of photos to give you an idea. The thumbnail shows a partial scrap wood mock up, there is also an attached image that shows the finished t&g pillars for the front of the flybridge.

I'm leaning towards just epoxy alone and fillets.....

MIA

[PAR]

If you use 5200, you'll have a flexible attachment, then cover this with rigid epoxy fillets?

[missinginaction]

Makes sense to me PAR. That's why I asked.

Thanks,

MIA

[Deering]

Not sure I see the advantage of a flexible joint if you're going to override it with a glassed epoxy fillet. Any reason why you need flexibility in that joint? I'm thinking epoxy fillet with glass tape, but maybe I'm missing something here.

[missinginaction]

Actually, I'm probably the one who was missing something when I posted the question.

After looking at the prepped parts there is no reason for a flexible joint. The fillet would eventually fail anyway. So 100% epoxy it is.

You ever have one of those "what was I thinking" moments?

[Landlubber]

"I'm leaning towards just epoxy alone and fillets....." you are such a clever fella

[BATAAN]

100% epoxy with filleting additive. Saturate everything with epoxy inside and out and paint with Awlgrip (tm) or equivalent. I built a beaching skiff this way 15 years ago and it's still going strong after hundreds of rocky BC and WA state beaches.

[pauloman]

use a flexible epoxy paste like QR 2400. Prime with solvent thinned epoxy or a moisture cured urethane. Then ideally a flexible epoxy paint with walnut non skid - then enamel or latex topcoat for the right color.

[PAR]

Sweet God, don't thin the epoxy folks. Why are people still recommending this? Please folks, read up on the testing and industry trends and DON'T thin your epoxy. It's not polyester and it doesn't act like the polyester molecule does. A lower elongation formulation (like G-Flex) does have some merit, but I'd recommend against it, as the rest of the structure is going to be stiff. If you want an isolation gasket (low elongation membrane) separating the two elements (also an idea that has merit), then using a proper bedding, under a flange and mechanically fasten it.

[pauloman]

thinning epoxy more than a few % does reduce its properties - and the added flex it provides is sort of proof also of the weakened properties.

However, thinned epoxy is fine if not being used in structural applications. It does reduce its waterproofing properties a wee bit too, but more than makes up for it with better penetration and bonding. Most so called penetrating epoxies are nothing more than solvent thinned epoxies. - Don't use them in confined spaces where the solvents have no place to flash off!

So adding solvent to epoxy will 1) add some flex - maybe only for a year or so, 2) lower visc, 3) extend pot life (important in very hot weather), 3) provide a wee bit of penetratiion (this is important as wood expands and contracts with moisture, while epoxies and just about all other things expand with temperature. In cold, wet conditions the bond between wood and epoxy is under extreme stress.

It should be noted that solvent based epoxies were the norm a generation ago and are still somewhat common outside of the boat building industry - and the basis for most penetrating epoxies and so called water based floor epoxies (which generally have a lot of voc solvents besides the water).


It should also be noted that some (not all) of the NSF 61 approved epoxies (that means approved for potable water - are solvent based).

Bottom line - don't thin your epoxy for structural applications or in confined sealed spaces, but for other applications thinning your epoxy can be as useful as thickening your epoxy is other applications. Sort of like booze - in some cases adding ice to the drink is a good thing!

The world of epoxies is much larger than the marine epoxy resins known to the forum 'experts' and includes such things as apply underwater epoxies (for piers, sinking boats, etc), water based epoxies, rubber epoxies, kevlar (tm) reinforced epoxies, novolac epoxies, coal tar epoxies, etc.

[PAR]

Thanks for your "qualifier" Paul.

Originally Posted by Pauloman

Bottom line - don't thin your epoxy for structural applications or in confined sealed spaces

Testing has repeatedly proven that substrate penetration has absolutely no bearing on waterproofness. It's the quality of the coating, not the penetration that makes things water proof, assuming sufficient film thickness.

I'm familiar with most of the resin types, though we don't talk about them much here. In the context of a boating forum, I think this is a reasonable think as well.

As to thinning epoxy (again) there are occasions that it can be useful, but I don't think there are any novice builders that need or should be attempting this in anyway. Of those that have an understanding of the chemistry involved, nonreactive modifiers and temperatures are a much better way to control viscosity and "penetration".

For the last 20 years "penetration" has been a marketing tool, more then anything else and not worth the BS it's built on. Don't even get me started on the Smith brothers.

[BATAAN]

PAR, Smith Brothers used to be in a WW2 barracks alongside the freeway in the east bay, on the mudflats, and had just started hawking their penetrating epoxy sealers and other stuff. A couple of us went there one day in the 70s for some polysulphide rubber for something and I got a look at the most disorganized, smelly, leaky, disastrous excuse for an industrial operation I have ever seen. The fumes made the windows rattle.

[pauloman]

PAR - we are starting to come to common ground!

I too have trouble with 'those other penetrating epoxy folks' - I dare not mention their name - the call out the lawyers on everyone that says anything. Their product is 69% solvent! (very little epoxy).

Epoxy has big molecules and thus is strictly a surface bond thing. That is why epoxy garage floor paints often peel - trying to bond to a dirty or weak surface. Adding solvents tend to thin the epoxy and soak into the floor/cement a little bit.

My factory chemists and formulators generally will accept thinning up to about 5 - 10% of the PART A only (before they worry about the lost properties). Of course in many situations it is OK to lose some of the high property values. (There are many marine epoxies and raw unformulated resins packaged as marine epoxies on the market because boat building is a relatively easy task for epoxy).

from experience I can say that when I do long term epoxy tests on plywood etc. - paint on the epoxy not all the way to the edge of the wood, I find that the epoxy will start to lift/peel on the edges (were the epoxy ends) after a year or so. When the epoxy is solvent thinned this is reduced. Granted, absolute max penetration with plywood would never be beyond the first glue layer.. And even with 100% solvent on good 'real wood' the solvents only soak in a tiny amount. Note that because of this 'reality' the term penetrating epoxy is rarely used these days. Now the words are "tie coat, bond coat, sealer," etc.

The best way to induce penetration with epoxy (as other suggest) is to warm the epoxy to thin it and warm the surface it is to be applied to. As the surface cools, any trapped air will contract and draw in the epoxy with it. This can be a real issue with cement floors which can release air as they warm up during the day and release air, forming bubbles etc in the still liquid epoxy top coat paint.

Note that when applying an epoxy barrier coat on a hull in 85-90 degree temps, even using a slow epoxy paint, it needs to be solvent thinned or it will 'ripple' on the very hot hull.

Keep in mind too that when solvent thinning epoxy, some of it will flash off and some of it will get mixed into the epoxy crosslinking (which will degrade the epoxy).

Again, never solvent thin your epoxy when using it in any sort of structural setting or in a confined space (ie. core repair without removing the fiberglass deck skin, etc.) but for sealing the surfaces of a cheap non marine plywood surface (without using fiberglass cloth) thinned epoxy will work (to some degree) without the drips and sags that happen when thicker 100% solvent free epoxy is used like paint on a vertical surface.

For sealing plywood, if a natural finish is NOT desired, two coats of a pigmented moisture cured urethane (MCU coatings) is easier and perhaps cheaper too.

I think we have reached common ground and shared a lot of epoxy info with the forum members. Perhaps time to put this to bed.

paul oman
progressive epoxy polymers inc.

[PAR]

You and I agree on many themes, most of which I've repeatedly shared with the forum, particularly my contention of the "hot on hot" method of sealing wood. I still don't think the novice or occasional user should be thinning their resins, but given experience, some understanding and very strict percentage guides some thinning can help some situations.

A number of years ago I did some physical properties tests on various solvent percentages and since this exasperating disclosure (to me at least) I've warned against any thinning.

In short I don't recommend more then 5%, regardless of the "special blend" many seem to dream up. I do have a few different formulations, for different situations, but most of this is repair and restoration work and to address special circumstances. The basic problem I've seen, is one guy cuts his resin 5% with acetone and another decides that 10% is going to be better, while other homegrown "alchemists" cut it even further, then ask their kids to join them on launch day!

Originally Posted by pauloman

. . . when I do long term epoxy tests on plywood etc. - paint on the epoxy not all the way to the edge of the wood, I find that the epoxy will start to lift/peel on the edges (were the epoxy ends) after a year or so . . .

If you're not encapsulating the test sample, you're exposing the epoxy to environmental changes and variables in the outer veneer, which will quickly test the sheer strength of the coating, particularly near the raw wood/epoxy interface, where the most dramatic differences will occur. Thinning epoxy more then just a few percent is just turning it into a paint, with the solvent becoming the vehicle, which commutes the resin into the substrate a bit, then flashes out. It's no longer a truly moisture vapor resilient coating, but something less, which is dependent on the solvent(s) employed and the percentage of resin dilution.