Est. 1998 — 28 years of woodworking knowledge
Question for John and others familiar with Epoxy
Barry Irby
Down below John in NM mentioned using less hardener in epoxy to slow the set time. Somewhere along the way someone mentioned using less hardener made the set Epoxy harder. Then there are thickeners and colorants.
Some of these things seem counterintuitive. Less hardener makes it harder???? Adding thickener makes it stronger???
Where can I learn more about all of this?
Re: Question for John and others familiar with Epo
Mark Mandell - Gone Round In New Jersey
Call the guys at Total Boat.com
Re: Question for John and others familiar with Epo
Cameron
I'd be very careful with this topic... As a chemist I cringe when I see advice suggesting manipulating the formula of a reactive mixture. Epoxy is typically a 2-part system, the base reactive epoxide and another species (usually an amine) that opens the epoxy ring and crosslinks the whole thing into a network. Disrupting the ratio of end groups too much will result in unreacted material and is almost always the reason for system failures (others usually being surface prep and incorrect temperature.)
Re: Question for John and others familiar with Epo
John in NM
I would only recommend that for West System, although I'm sure there are other similar epoxies. With one to one mixes its not a good idea.
The only reason it works (or appears to work, in deference to Cameron's comments) is that the product develops significant heat while curing. Heat accelerates the cure of epoxies in general, so reducing the catalyst is intended to slow down a reaction that you think will be problematic, preventing cracking and boiling. Without a doubt the West System product we were using was not designed for such massive applications 
In general I agree with Cameron that messing with the mix of two part systems is not going to give you better results - it always slows the cure and often makes a weaker solid. In the case I mention, it was done to prevent worse problems.
In retrospect, these days I would probably be looking for a better filling agent than what we were using for massive voids (this was 25 years ago). It was harder to find good solutions then, even though the internet was a thing, it was not yet in widespread use. I bet even then all the boss had to do was call up Gougeon and ask what to do about it. It seems like the better solution would have been a slower cure hardener or a retarding additive.
Re: Question for John and others familiar with Epo
Ralph Lipeles
"Hardener" is a somewhat misleading term. It is a chemical that initiates the reaction that transforms the 2 parts of the epoxy into one that will adhere and set to a solid. The amoust of hardener to use is stated by the epoxy maker. If the user changes the ratio it usually causes the cured epoxy to have less strength or to have poorer adhesion to the parts it is being usen on. .
One thing about epoxies that has not been mentioned is storage of the epoxy. Storage can extend it's life. When I worked at a company that made high tech radar for the Navy, we used lots of epoxies. At that time the mil specs required that epoxies had to be discarded after 6 month from the time that the epoxy tubes/cans were opened. I would bring the 6 month, expired epoxies, home and put them in the freezer. That required a little planning ahead so that when you used the epoxy it was defrosted and in a useable viscosity for working. I still use some epoxies that I froze 30 years ago.
Re: Question for John and others familiar with Epo
Ralph Lipeles
For parts that are difficult to hold in place, a faster curing epoxy should make the fixtureing problem less severe.
not a good idea
Bill Tindall, E.Tn.
Screwing around with deviations from directions when it comes to doing polymer chemistry will result in worse adhesive/finish/whatever properties. The variables to not be messed with are catalyst amounts and in the case of epoxy the ratio of polymer (the epoxy component) to the crosslinking amine (hardener). Short change the cross linker and the result will be less tough, at least.
To slow down polymer chemistry (cure) cool the reaction.
Adding fibers or other strong component (thickener) can improve strength. Think fiberglass reinforced epoxy for a boat. At the optimum ratio the strength is determined by the additives strength and the epoxy becomes the binder.
The interaction of all these variables can be complex and lead to changes in the cured polymer properties for better or worse.
chemistry error
Bill Tindall, E.Tn.
Heat accelerates the cure of epoxies in general, so reducing the catalyst is intended to slow down a reaction that you think will be problematic,
Epoxy "hardener" is not a catalyst. It is a molecule that reacts with the epoxy component to crosslink the epoxy polymer. It is an essential component of the formula. The formulation is designed to have an optimum amount of crosslinking to get optimum properties. Changing its ratio will be like altering the amount of cement added to sand and rock to make concrete.
Can you change hardener ratio? Sure and the result will be something that gets more of less hard. Maybe hard and tough enough for the application. Furniture is always a good lab in which to do chemistry experiments on adhesives and finishes. 
Sure...
John in NM
That makes more sense than my explanation. One trouble with this stuff is that it is less than obvious to us casual users. The tricky part is remembering the expert explanations later
In the case of filling large voids using something like West System, if you have problems with cracking or boiling, using a short measure of the hardener will avoid those problems. While the advice is less than ideal in many respects, but in a production shop it is a reasonable substitute. Usually management doesn't want to wait for the proper product to be found and purchased, you don't want to waste lots of time filling little bubbles or cracks later, and the hardener reduction did not seem to be too detrimental in practice.
Having since used some very fast and hot urethane resins, I think that "boiling" could instead have been absorbed water coming out when it got too hot. The urethane resins got that hot regardless so you had to be very careful to use new, clean, plastic mixing cups, but I only recall West getting really hot in large, bulky shaped pours.
Wow, Thanks, I would think...
Barry Irby
the manufacturer would state the optimum ratio, and I am inclined to go with that. The idea that some of us basement tinkerers could casually improve on the product seems unlikely.
I wanted to weigh the components but the seller said to stick with volume measurements. My real question here was to find out how sensitive the measurements were and if I were to ere would it be better to long or short on the hardener?
Another question: Once I fill the void and even mound it up a bit it seems to shrink. Is that so? Since it has low VOC's or none, it seems there is nothing escaping so what accounts for the shrinkage?
What additive to avoid? Trans tint dye?
Re: Wow, Thanks, I would think...
John in NM
How best to measure depends on how you want to do it. West sold pumps that kept the right proportion with a single pump of each - volume basically. If you're measuring manually by pouring into two cups, I would do weight since it seems easier to repeat accurately. Postal scale from Walmart was under $20 last time I bought one, and measure to the gram easily.
For dye we used to use ProLine universal tint from the paint store. Doesn't take a lot of tint if you're going for black, so it doesn't affect the epoxy much. I've gotten TransTint to mix into 5 minute epoxy by adding a little acetone, but it makes the epoxy too rubbery to be useful.
The shrinking could be that the epoxy is more dense once polymerized or whatever it does (Bill or Cameron would know better than I do about that). Since it can get hot while curing, you can also attribute some of the shrinkage to it cooling down. How much is caused by either of these I couldn't tell you.
Re: Wow, Thanks, I would think...
John b
Raka Epoxy,. has an online epoxy manual that will answer all the questions that you have asked
Re: Wow, Thanks, I would think...
William Duffield
A very useful information source from the Gougeon Brothers, developers and users of West System.
I have been successful using TransTint, in small amounts, one drop in one pump resin and one pump hardener. I found out the hard way (it didn't 
) that you must thoroughly mix the resin and hardener before adding the dye (alcohol based). Otherwise, the dye and carrier get in the way of the polymerizing. I suspect using a dry powder pigment would be more lenient in mixing schedule, but I can't guarantee that.
I once had a surprising overheating event with a half cup batch of West that I'd used most of on a very hot* summer day. I set the cup down on the rock driveway. when I turned around to check on it a few minutes later, and it had swelled up to the shape, size and color of a candy apple, with steam coming out of the middle of the top, where the stirring stick was still in the goop. Most of the reason for the color was the hardener, which was old. Evidently old hardener still works as well as fresh, even if it has darkened substantially and become more viscous.
* That's "very hot" for coastal New Jersey, not New Mexico. 
Hehe...
John in NM
Not that hot where I live now, there's still a bit of snow from that storm that whacked Texas That was an extra cold storm, but we usually have snow here for a few days after a storm passes through. Rarely hits 100 in the summer.
Most of the work like this I did when I was in Tucson, they used to put the 2 hour epoxy out in the sun in the summer - cured in about 15 minutes! That may have contributed to the bubble problems, I've seen it smoke pretty good on a summer day.
I rarely did that myself, but the shop had a kid that did the filling and milling for the assemblers so you couldn't always control what you got to work with. A bunch of us would do our own milling if the boss wasn't around to give us dirty looks
Why....
Bill Tindall, E.Tn.
using a short measure of the hardener will avoid those problems.
That is because there is less cross linking reaction occurring which will lead to less than optimum properties if that is important.
It was never clear why a large volume of epoxy was being used for whatever you were doing. This application is not suitable for typical epoxy because of the large amount of heat generated by the cross linking reaction. In a large volume there is poor conduction to allow the heat to dissipate. The heat itself will lead to loss of properties. If I had a large hole to fill I would be using styrene resin to do it with a gob of cheap inert filler(Bondo).
True...
John in NM
Less crosslinking leads to less heat. Presumably less hardness, but also no cracking and bubbles, thanks to the reduced heat.
Slightly softer fill material didn't seem to be a problem, but cracks and bubbles to be filled later required more time spent by the cabinet makers and finishers. If the cured filler were really soft and rubbery (like 5 minute epoxy with acetone and Transtint added) then I doubt the practice would have continued past the first attempt. It was less than ideal but apparently good enough.
I honestly can't un-recommend the approach because it works ok, with one product, in spite of being a chemically illiterate short cut. The discussion has provided a much better context of the risks of using it however, which is always useful.
As to the big voids, Sonoran mesquite often has them, especially around crotch wood. Mesquite stuff sold well in Tucson. It made less sense in a small shop to have two different filler products than have an informal practice of short hardener mixes for filling huge holes.
Sometimes the boss liked to shape a block of scrap mesquite to fill the hole with epoxy around it, but I felt that only looked good if it looked real - like when you'd see a branch section coming out of one of the big cracked voids. It was hard to do well, so it was rarely done.
Obvious question is why anyone would try to make furniture out of this stuff at all.... well, it sold well. The wood itself is pretty for the most part, though I personally don't care for the cracked and filled look.
Re: Mesquite
Mark Mandell - Gone Round In New Jersey
A friend gave me a nice wet 2' by 15" log section.
So many checks and cracks developed that it got relegated to the grill smoker pile.
Although I work with cracks in turning, that log was beyond even my willingness. Smells great doing lamb.
Re: Mesquite
John in NM
I could see a log section quickly turning to useless cracked pie pieces The stuff does not have a lot of tensile strength. Makes good barbeque wood though, one of the guys at the shop in Tucson used to save all the scraps he could (sans epoxy) for making Carne Asada. Just don't stick it in the fireplace - the one time I tried that the house smelled especially smokey for about a week afterwards.
Re: Mesquite
Mark Mandell - Gone Round In New Jersey
I keep enough for a BBQ in a ziplock with water; adding more wood as it's used. When the water gets dark, I simmer it to reduce by 75% and then bottle the result for kitchen seasoning.