Simple Epoxy Questions (epoxy as adhesive)

[James Jayko]

Hey all, I'm about to start gluing up a Morris chair I'm building. It feels like its going to be a relatively stressful glue up (see the dry assembly photo below). I'm going to do the leg assemblies individually and then connect the two sides as a third step.

I figure I should use epoxy given the amount of joints and the long working time, but I've got some basic questions, given that I have only used epoxy a handful of times.

1. Is there any real difference between the popular brands (System3, West Systems, Total Boat)? I see Total Boat the most on YouTube and whatnot, but I assume that's just that they choose to sponsor more content creators. But I don't know.
2. Do I need to use a thickener like Total Boat's silica thickener? Seems like it would probably stick to surfaces better as I'm getting these pieces assembled, but the joints fit pretty well so I'm not sure.
3. I'm going to try to be careful and only do the mortise to avoid squeeze out as much as possible. The chair is cherry; would you add a tint to whatever epoxy you're using? Or do you do both mortise and tenon on structural joints and just clean up with acetone? How would you attack this to ensure structural stability? With epoxy, I only get one shot for sure.
4. When looking at Total Boat's website, I see a "slow" and a "fast" curing option, but I can't find a clarification of what slow and fast actually mean (open time, clamp time, etc). I can't imagine it would take for than ~20 mins to get the legs together, but extra working time can't hurt given that I'm in no rush and if I had to leave it for 24 hours, that's fine.


I do realize that I might be wildly overthinking this. But I'm listening to any "epoxy as adhesive" advice you've got!

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[Richard Coers]

I'm not a fan of epoxy for joinery. Epoxy needs a fat glue line to reach full strength. Have too tight of joint and it will starve the joint. One company recommends .002-,007" thick bond line. That means a motive and tenon joint would have to be .004-.014" loose. That could mean doing a little alignment with all the joints until the clamps go on.

[Michael Burnside]

For the most open time I use the standard Thixo product line. It will give you 20'ish minutes at room temperature before you start seeing it gel up and harden. If it's going to be a smaller piece or easy glue-up I use the 4-minute epoxy which is usually plenty of time for a given joint.

Some things you should remember:
1) Expoxy doesn't need extreme clamping pressure, it's surface adhesion is off the charts compared to wood glue, so don't go nuts and you'll have little to no squeeze-out - this is the biggest mistake most people make, especially woodworkers
2) Less is more. Just lightly coat the surface
3) Use blue painters tape on anything you think might squeeze out
4) If you're going to sand the area anyway it's not a big deal
5) You usually have more glue-up time than you realize with expoxy.

I recommend if this is your first time I'd test a tenon/mortise on a junk piece just to get a feel for it. I use epoxy often for outdoor furniture or tricky glue-ups...works great.

[Matt Carr1]

Rarely use epoxy but I have west system with the slower hardner. When I looked into all this before what I found was the difference in slow vs fast was something of 10min pot life vs 20 min, cure time is still similar. The silica thickener is to turn it into more of a gel. And lastly is it’s expensive. West systems has a flow chart of products and descriptions on their website. I have mostly used epoxy for gap filling.

[andy bessette]

...Epoxy needs a fat glue line to reach full strength...

Entirely false.

OP--never add anything to epoxy that is not specifically recommended by the formulator.

Epoxy is an excellent choice for this type of project, for which I would use slow hardener (there really is no need to rush). Both surfaces need to be coated. Squeeze-out is essential to be certain of having no voids, but needs to be cleaned up immediately. Colloidal silica is used where necessary to thicken the mix to prevent running and to better fill any gaps. Avoid heavy clamping pressure--it is unnecessary.

Suggest you first read "The Gougeon Brothers on Boat Building".

[glenn bradley]

I really only use epoxy as an adhesive for outdoor projects or joints that need to slide during assembly. The longer open time of slower set epoxies may be helpful but I try to design things so that I can glue up sub assemblies that then glue together to make the whole piece. I find this less messy than gluing up a dozen piece assembly at once. For sliding dovetails, epoxy is great. It acts like a lube while sliding the parts together where water based glues can cause swelling, binding, or can just plain grab (by design) too soon. Take care that the videos you are watching are talking about adhesive use and not river tables or encapsulated slabs. The epoxies for those uses have different characteristics than simply sticking things together.

[Michael Burnside]

I'm not a fan of epoxy for joinery. Epoxy needs a fat glue line to reach full strength. Have too tight of joint and it will starve the joint. One company recommends .002-,007" thick bond line. That means a motive and tenon joint would have to be .004-.014" loose. That could mean doing a little alignment with all the joints until the clamps go on.

Not my experience at all. I have no idea where you're getting that from, it just doesn't make any sense. At least not with Totalboat in context. There is little to no difference in joinery difficulty. In fact I would argue expoxy is better as it has a tendency to prevent the joint from swelling. I'd use it more often if it wasn't so damn expensive!

[Holmes Anderson]

You probably don't have to glue all those joints at the same time so maybe it isn't as stressful as you imagine. I only use West System so couldn't tell you if there's a difference but I always use a thickener unless I am just wetting out a hull. West's 405 filleting blend is brown so it blends better with wood than microfibers or silica. You can also add pigments. Sanding is a pain so tape and then use denatured alcohol and your finger in a nitrile glove to work squeeze-out. The alcohol acts as a sort of lubricant, preventing the epoxy from sticking to your fingers. I never worried about minimum thickness for a joint. Solid mahogany can be laminated with epoxy and 405 adhesive filler for rub rails, for example, with a glue line similar to TB3 glue. The pot life for the hardeners is listed on the can. Fast kicks very fast. I mostly use epoxy for building boats so couldn't say exactly how those m&t joints will accept the epoxy but I wouldn't thicken to peanut butter consistency. Maybe more like mayo consistency. You could use West 105 resin with the 207 special clear hardener and wet out the entire chair to make it waterproof and then apply marine varnish (epoxy will be damaged by UV without protection).

[Kevin Jenness]

1. Is there any real difference between the popular brands (System3, West Systems, Total Boat)? I see Total Boat the most on YouTube and whatnot, but I assume that's just that they choose to sponsor more content creators. But I don't know.
2. Do I need to use a thickener like Total Boat's silica thickener? Seems like it would probably stick to surfaces better as I'm getting these pieces assembled, but the joints fit pretty well so I'm not sure.
3. I'm going to try to be careful and only do the mortise to avoid squeeze out as much as possible. The chair is cherry; would you add a tint to whatever epoxy you're using? Or do you do both mortise and tenon on structural joints and just clean up with acetone? How would you attack this to ensure structural stability? With epoxy, I only get one shot for sure.
4. When looking at Total Boat's website, I see a "slow" and a "fast" curing option, but I can't find a clarification of what slow and fast actually mean (open time, clamp time, etc). I can't imagine it would take for than ~20 mins to get the legs together, but extra working time can't hurt given that I'm in no rush and if I had to leave it for 24 hours, that's fine.

Marine epoxies are generally equivalent.

You don't need thickener for well fitted joints. If you do use it, wet out the surfaces without it, then add thickener to the mix and re-apply. Cellulose microfibers are good for gap filling, colloidal silica is good for thickening but it is hard to sand, and additives like microballoons make an easily sanded lightweight mix.

Wet out all the surfaces. You can remove all but a thin film on the tenons with a scraper.to limit squeezeout. If the parts are prefinished light squeezeout can be popped off after cure. Vinegar will cut freshly mixed epoxy as will alcohol. When the mix starts to get gummy acetone is needed. Butt joint gluelines are typically darker than pva due to absorption into endgrain. Tinting won't do much if your joints are tight.

I mostly use fast hardener unless temps are above 70*F. It's mostly about potlife. Once the glue is spread it gradually thickens up and you can typically still pull joints together within an hour. If you have an extensive glueup do it in sections and mix several small batches of epoxy rather than one big one. The reaction is faster in a concentrated volume so pouring the mix into a shallow pan will extend working time.

You may find that by breaking the assembly into sections you don't need epoxy.

I don't find that epoxy requires fat joints. Pvas work well with a .002" glueline as does epoxy. What it does need is a rough surface. West System recommends 80# for mating surfaces. I had several failures on dense woods with cleanly milled surfaces before learning this.

As with any unfamiliar procedure, do some experimenting before committing to a major project. Wear gloves as epoxy is an irritant and some people develop a serious allergy.

[Scott Bernstein]

I like to use epoxy for joints in which I expect to have some gaps, and for projects where the glue-up is complex or tedious where I want some extra open time before it sets.

Unlike regular wood glue, a slight gap - or even a larger one - does not diminish the overall strength of the joint with epoxy. Of course the gap may not look great, but at least it will be strong.

Some epoxies are thicker than others. I find System Three T-88 to be very thick and it takes a long time to fully cure. West System epoxy is much thinner and takes less time to cure. If you find a particular epoxy is too thin and easily runs, you can certainly add an appropriate thickener or structural filler first.

[Richard Verwoest]

Ambiant temperature also plays a part with the open time o epoxy. Have you considered using hide glue? I build a similar chair a few years back using hide glue. I had the same fears you did. Hide glue worked great. And I used the ready to use hide glue.

[James Jayko]

Ambiant temperature also plays a part with the open time o epoxy. Have you considered using hide glue? I build a similar chair a few years back using hide glue. I had the same fears you did. Hide glue worked great. And I used the ready to use hide glue.

doesn’t hide glue go off VERY quickly?

[Tom M King]

If anyone ever uses the thickener for anything, wear a respirator. The stuff is really fine and light enough to float in the air. You don't want to breathe Any of it, and it will end up in the air around you regardless of how careful you are. It's Silica-not something you want in your lungs.

If you have never worked with epoxy before, I would absolutely advise against assembling a chair with it being your first use.

[Richard Verwoest]

No, hide glue has a solid 20 minute open time in my experience.

[Mel Fulks]

Richard , I suggest adding what kind of hide glue. The liquid at room temperature stuff sold ready to use from bottle is really slow set.
Hot hide glue granules are sold ,I think ,in 3 different strengths , I can’t remember if they have cure times.