Hi guys,
I have a very simple and almost trivial question but it is a problem.
I have mixed epoxy probably not exactly 1:1 and after a day it is
still very soft , it stays on your finger after you touch it .....
I just put this layer as reinforcement, parts were actually glued before applying extra
coat of epoxy.
I need to make these areas non sticky.
I was thinking about "painting" over it with Gorilla glue .....
Any ideas ?

ARB

Wipe off as much of it as you can, then wipe it some more. Then wipe with acetone soaked rag. Do NOT apply anything over it, as it will not be bonding to anything, and you will end up with an even bigger mess.

Thanks Deadeye,

Do you have any hints how to apply equal amounts of epoxy to mix it?
Mine is GP and hardener is a little thinner than epoxy itself so it is hard to squeeze proper amount.

Thanks.

ARB

It's not cured and therefore, isn't "bonded". Get it off and start all over. Epoxy isn't that exact in the mix, and can be off a little bit either way and still cure properly. If you were anywhere close to 1:1 and it didn't cure, throw it away and buy some fresh. Be sure the epoxy is at room temperature when you use it. The pieces to be put together should also be at room temperature. You can use graduated measuring cups, like come on the top of Pepto-Bismol bottles, to measure accurate amounts. These should also be available at your LHS in guantity.

Dr.1

For smaller mixes, I use a piece of heavy paper or cardboard, squirt the thicker of the two on first, then squirt the thinner of the two to about the same size. Mix away. If it's 5 minute(for foam) let it 'rest' for thirty seconds then mix it again and apply. If it's thirty minute (what you should use for wood) let it 'rest' for a few minutes then final mix and apply.

Thank you guys, will try to be more careful next time....

ARB

I don't know that I'd let 5 minute "rest". When it says "5 minute" on the label, it doesn't mean "5 minute and one second". The epoxy will begin to set up and become too thick to use before the 5 minute mark is reached. Work FAST.

Deadeye is right. Use 5 minute only for fuelproofing or foam, and 30 minute (or longer) for wood.

Dr.1

I bet it does kick off fast in SC. Here in the frozen north we get 10 minutes easy with 5 minute.

Dr1 Driver is correct that the mix ratios of epoxies don't have to be exact , just close.

Most of the time if your mix ends up in a sticky mess is because the material was not inducted(mixed properly. Too many times I have seen folks measure quantities of epoxy and then just give it a casual mixing. Epoxies have to be mixed well for best results.

Epoxies cure by catalytic reaction they don't dry, they cure!

The time given on a epoxy ie. 5 minute, 30 minute, 24 hour are "pot life" times at a controlled benchmark temperature like 77 degrees F. , its not the cure time. There are only two things that effect the cure time and pot life of epoxies other than the chemistry of the resin and the catalyst. They are temperature and mass. Meaning if you will have quicker cure if the epoxy is in a warmer enviroment vs a colder one. A thicker layer of epoxy will cure faster than a thin layer because it generates more heat in the the reaction its mass is greater.

Below around 50-55 degrees F the calalytic reaction almost goes into remission or stops with most epoxies unless its a low temperature product.

Lastly the strength of most epoxies even the so called 5 minute versions is far greater than the fiber strengths of what we normally bond building or repairing model aircraft. The only exceptions might be hard maple or oaks.

A slow set epoxy generally gives a stronger bond because it has more dwell time on the surface area for the wetting agents to penetrate the substrate better thereby increasing strength. The fast set epoxies give more of a surface bond. If your working with softer woods this is not an issue as the shear values generally exceed the fiber strength of the woods anyway.

What do you mean by proper mixing?
I was stirring it in a little medicine cap for about 1 minute.....
How long would I have to do that to obtain proper mixing?

ARB

I suspect your proportions were off. For small batches I squirt two ribbons of approximately the same length and thickness measuring by eyeball and this always results in a successful cure. Make a test batch being careful to get equal amounts and mix it well to see if it cures properly. If not, throw all of it out.

Also, painting epoxy on a surface really doesn't add much reinforcement; it just adds weight.

ARB,

Did that medicine cap have a waxed seal on the inside top? Mixing epoxy on waxed paper or waxed cardboard will alter the curing process. Many times to the point where curing doesn't take place.

Just a thought.

Ed,

I think THAT was the case ....unbelievable, I have never thought about it.
All my previous mixings were done in servo box top ( plastic ) and for the first time I have
tried to use the medicine cap because it has a nice size and you can mix it better ( smaller area versus hight).
It IS waxed cardboard or waxed paper the cap is made of.
Now , the most interesting part, would wax kind of mix with the epoxy and hardener to stop it
from curing?
I am almost positive you found the cause.... unbelievable, I would never, ever think of it.
I was so surprised because I have mixed epoxy hundreds of times before but never had that problem before.
Will remember it from now on......*smile.

Thank you very much.

ARB

Mixing most epoxies for a minute is more than adequate. I normally don't mix in a cup, I prefer to mix on a flat surface where I can mix the components folding them into together by scraping them up with the edge of my mixing tool, then spreading the material out again ensures a good mix.

A good mixing tool is a artists pallet knife sold in art stores. I like one about 1/4" in width that is flexible with about a 4" blade. I use the same knife to apply the epoxy it allows for pretty detailed application even in most tight areas.

Steve,

Thank you for the hints.
Now I know it was not the mixing time but the wax !!!!!!! Still can not believe it would affect the
process to that degree.

Pallet knife you are talking about sounds good but when you need more volume, I was reinforcing
ARF's canopy holding pegs on GP Yak 54 25%.
I understand and like your method for larger amounts.

ARB

I doubt that the wax in the cup was the problem. If proportions were close and the epoxy was mixed well it should have cured. It is doubtful that you would get enough wax migration into the epoxy to cause problems. Wax however is a releasing agent for epoxies and is often used in mold applications to prevent sticking. Any containates on the surface of the areas to bonded like wax, oil, grease, dirt, or dust may prevent a good bond.

I am not an expert here ..... but where would Ed get the idea from if it did not happen?
I perfectly understand preventing from sticking ability though.
I hope Ed will reply with more info on above.

ARB

arb6595,

There is alot of misunderstanding and myths out there about the proper use of epoxies. I say this because we deal with application problems everyday in our business where we manufacture and sell epoxies.

I use the pallet knife to mix from very small amounts of material to pretty large quantities. When it gets to really large mixes we use a putty knife again using a flat surface. For most mixing I use a 5" X 7" pad so I can get a clean mixing surface for each mix. While You Where Out phone message pads work well for small mixes.

Hope this helps!

Steve,

I understand about the misinterpretation and myths but what really puzzles me is that when I was squeezing epoxy and hardener to
the cup I was rather trying to put more hardener then epoxy and got these weird results.
I have been mixing it many times with off on both sides but I can not imagine being so much off to the point that
the mix did not cure at all, in the morning , after 10 hours it was basically in a liquid state which means I would have to be waaaaaaaay off.
This is what puzzles me the most.

Thank you very much for all the info.

ARB

ARB,

Now I think we have the answer to what really happened. Epoxies don't work like fiberglass where if you add more hardner you will get it harder and hotter even to the point of catching fire.

With epoxies the more hardner you add the softer the cured material will be to the point where its gummy and won't really cure. The less hardner you add the harder the cured epoxy will be to the point where its too hard. What we do as manufacturers is to give you a mix ratio that provides a very strong bond, but is not so hard that it will shatter if shocked or stressed.

We get alot of home chemists that we have to deal with. Most of our product is supplied to the furniture repair industry. Some of these folks insist on second guessing us and often get failures.

Its also possible that you got mixed up and mixed two portions of A or B together it does happen.

Have a great Thanksgiving! PM me if you have other questions I'll be out of touch for a day or so but will get back with you.

Steve

Steve,

Now that I have said I was trying to put more hardener and it works the opposite way .... everything is clear ...*smile.

I would bet the money that the more hardener-the harder the final product and the faster the mix will cure ...... you can learn
something every day.

Thank you very much for all your time and have a great Thanksgiving too.

I am going to add you to my buddy list if it is available here on RCU.

Thanks again.

Andrzej

While we're on the epoxy subject I have a quick qustion. I mixed up some 10 min epoxy and thinned it with rubbing alcohol to use for applying some fiberglass tape to the inside of a fairing, it was fairly liquid for about 2 to 3 minutes then almost instantly it fired off into a hard rubber ball, so fast the second time that it actually captured my paint brush.

Question, what is the best way to thin epoxy? I have always read of using the alcohol method and number two, does the alcohol make the epoxy fire off quicker? If it does then I just need to get a longer working time epoxy. No problems with what I did just an observation and a little aggrivation having to mix it three seperate times to put on about 8" of 1" wide tape.

I have found by experience (or should I say inexperience?) that mixing in a waxed container or on wax paper will most often cause a bad cure and sometimes it will never completely cure. The old Hobby Poxy instructions actually warned against using a waxed container.

Rubbing alcohol has a LOT of water in it and may cause a bad (or no) cure. Use only a very little or use denatured alcohol (available at some LHS and most drugstores). Use a longer set time type and use a LITTLE heat to thin it. Caution: Heat will also make it set up quicker.

Dr.1

My comments below refer to GP epoxies such as those that we use in this hobby
Epoxy cures by cross linking of carboxyl groups - not by catalytic reaction. That is why it is important to use exactly the correct mix. Way back when i dealt with them on a daily basis, I used to say to customers that thought that if they wanted them to cure fast by adding more hardener, "Why not use just hardener then?"

The next point is that the chemical reaction wil generally ceas below around 15 deg C and be inconsistent (due to the speed of the reaction) above around 35 Deg C. (Ambient Temperature).

Many of my old clients that had complaints would complain that they mixed the product and it hadn't cured overnight - Usually on a cold night and with the product outside in the garage or garden shed.

Waxed containers can be an issue if you are only mixing a small quantity - the wax from the container can enter the mix and act as a release agent between molecules thus stopping the cure from taking effect.

The best way to measure small quantites is to use either a set of small scales or use 2 teaspoons (one for each component). make sure that the entire amount is taken from the spoon to the mixing board.

So - in future - Measure it accurately, mix it thoroughly, make sure the temperature is within the correct range (Ambient as well as the temperature of the item being glued), do not use waxed cups and do not contaminate it with anything. you will have success each and every time that way.