From CLC's Tips for Boatbuilders (aka Shop Tips), available through the link at the top of every page on this website, under the section labeled Working with Epoxy and Fiberglass, in the section titled Epoxy Basics:

As epoxy cures it generates heat, which accelerates the cure time. If a large mass of epoxy is left to cure in a container, such as a mixing cup, it will harden very rapidly, often in a matter of minutes. If, however, the same quantity of epoxy is quickly spread out, as when coating a boat's hull, it may take hours to harden. In the first instance there is relatively little surface area and the heat generated is not dissipated and causes the epoxy to rapidly solidify. Conversely, the thin film of epoxy, with much more surface area, allows the heat to dissipate. When long working times are important, always get the epoxy out of the mixing container quickly. Another trick is to pour epoxy into a shallow bowl or pie tin to increase working time. Containers of curing epoxy can generate a significant amount of heat; always place extra mixed epoxy outdoors.

FWIW, you're in good company. My smoking hockey puck 19 years ago was 8 oz.

Live and learn,

Laszlo

There is SO MUCH information on CLC, and it is so spread out, so I missed this one. I am surprised that the manual, which has a section on epoxy, aimed at newbies like me, did not mention the issue or ways to deal with it.

Follow-up question: I glued two of the panels together with epoxy (in a smaller, 4-oz batch) that was in the process of becoming a warm hockey puck. I managed to finish the job, using my (gloved) hand to spread the thickening epoxy. All the surfaces that needed to be covered, were covered. Is this going to be a good joint? (This was a few hours ago, so I won't be able to check on the panel until I get back to my work area in a few days.)

Everybody needs a hocky puck. Sadly, I created a softball when building my dory. But so it goes.

As to your joint with thickening epoxy, you'll be the best judge of its strength. If you got the epoxy spread and into good contact with the wood before it became granular, then I think you'll be OK. Clearly, though, you'd like the epoxy to be thin enough to soak into the wood a little before hardening. If that hasn't happened, you may need to reinforce the area with "peanutbutter" putty or fiberglass cloth. Fortunately, these boats are really strong and epoxy / fiberglass construction is very forgiving. Plus, anything can be fixed down the road!   

We all start out knowing nothing at all about things we want to undertake. It’s the process of discovery that enthralls most of us, inspiring our continuing adventures as we gain knowledge and experience as we go, only to find new interests that keep us engaged as we start up the new learning curve once more.

I read a lot.

Here & elsewhere.

Also comminicate with folks doing stuff I’m interested in.

This forum is a good example!

As for mistakes? I understand they’re inevitable at the feet of the Gods of Learning Anything.

As for making good joints? Mix your resin & hardener well, then separate out a small portion to be used for wetting out edges. Use the rest for adding appropriate fillers to apply to the joint. It all cures mostly at the same time but the unthicked mix is best suited to soaking into edge and end grain to prevent mixed components from being sucked away from the joint during assembly.

Be aware too that epoxy ‘cooking off’ has been known to burst into flame.

(Maybe not MAS stuff though?)

Not the best guest to invite into a garage, basement or wood shop.

Why I suggest a starting-out-slowly methodology for folks unaccustomed to working with materials they’re establishing a new relationship with. Mix small batches for the first few sessions, a couple ounces at most. Progress to twice that once you’re confident you can use it all before it ‘clicks’.

I’d be wary of any mix that’s begun to warm yet not applied to your project. That warming indicates the reaction began well beforehand so the mix isn’t likely to behave the sameas a freshly-mixed batch.

   My thoughts......

 1. I've not had a hockey puck in decades. It was before I started using the pumps that measure the epoxy and catalist.  The heat and hockey puck are usually the result of too much...........usually too much catalist. How are you measuring your mix?

2. Never leave when epoxy is mixed.

3. Make smaller batches. A 12oz batch will result in wasted epoxy.

   4. Which hardener (catalist) are you using, slow, medium, fast?

>>The heat and hockey puck are usually the result of too much...........usually too much catalist.

Not with epoxy resins.

Catalysts are for polyester resin, hardeners for epoxy resins.

Catalysts trigger/accelerate a chemical reaction that would eventually happen anyway, without actually participating in the reaction. If you leave a jug of polyester resin long enough, it will harden even without the catalyst.

Hardeners provide the material needed for a chemical reaction to happen at all. Epoxy resin by itself cannot harden. At most, after many years it may get gummy or crystallize, depending on the temperature. It must have the hardener present for the reaction to proceed to hardness, otherwise there is no reaction. Think of the epoxy resin as a pile of bricks and the hardener as the mortar. Without the mortar the most you can get is a stack of bricks.

Since the catalyst accelerates the reaction, the more you add, the faster/hotter the reaction gets.

But the hardener amount doesn't control the speed of the reaction, the hardener type does. Slow hardener is like a bag of mortar powder, fast is like pre-mixed mortar - the former takes longer to become a brick wall than the latter. In either case, not having the correct amount ruins the wall.

For a given hardener type (fast, slow, medium), the reaction's speed is controlled only by the temperatures of the ingredients which is controlled with the methods mentioned above (container size/shape, total quantity of the mix, etc.). Never ever attempt to control the epoxy reaction rate by changing the amount of hardener. That will result in a bad batch that will be either too hard, too soft and still may catch fire if it's large enough in the wrong kind of container. Always use the manufacturer-specified ratios.

Laszlo

   And so far most of the conversation has been about glue joints - so for the newbie, some more advice.

Doing the fiberglass work with epoxy is much trickier than glueing, in my opinion.  Working in hot weather (mid-80's and up - or over 100 here in Texas) is the time to get careful!  Screw-ups are harder to fix and hide, and easier to make.  Plan your work well, do the job first thing in the morning, have the cloth all ready and smooth, keep your epoxy jugs in the air conditioned house until just prior to use, don't mix batches that are too large, mix thouroghly but quickly, use wide containers that maximize surface area to dissipate any heat, pour more out of the container onto the cloth to let it sink in and wet out while squeegeing areas poured a minute earlier keeps the epoxy cool and slows the hardening - but not too much ahead of yourself, as having epoxy start to set prior to working it into the glass for a good/complete/non-lumpy wet-out is a terrible thing to do.

NEVER try to continue to wet out glass with epoxy that has ceased to be nice and runny.  Trash the batch (cry over the lost money, but not too much, as is it was a necessary thing to do and easier than fixing a messy glass job), then start with a fresh mix.

Best advice if you haven't done any epoxy glassing in warm weather is to do some smaller pieces first so that you get a feel for the process and how much time you have.  You have to plan all moves in advance and can't handle interruptions.  Story: One time I answered the phone in the garage with my gloves on and epoxy cup in my hand, thinking I would just say "too busy to talk now" and hang up - well it took me a minute too long to get that said, and left me with a wrinkle in the glass that had firmed up while I was being stupid.

   One last tip for the inevitable epoxy mistake: you'll be amazed at how easy it can be to remove fiberglass cloth and epoxy -- even days after hardening has begun. A heat gun or a hand-held hairdryer is the one essential tool. Thoroughly heat up the affected area (being careful not to scorch the wood) and the epoxy will then usually come off cleanly and quite easily. I've had to fix some pretty bad blunders using that technique, but it works a trick!

Thanks for the advice. I have just returned to my work area (it's 2 hours from home). I'll start a new topic with followup questions.