epoxy ratios

I blend resin and hardener 50/50 but started wondering, what happens if you do 70/30 in either direction, or even try to let either dry by itself? I could have sworn I heard someone talking about using one of the two for some kind of treatment or surface preparation, though I cannot find any evidence of that now.

How careful are most people about hitting 50/50?

Resin ratio depends on the particular system you're using, and following the directions for that particular epoxy is critical to getting a proper cure.

It's also important to follow the directions for each particular combination -- for example, with a given resin the 30 minute hardener might be 3:1 the 120 minute hardener is 5:1 with the same resin.

-Kevin

Ratio depends on the epoxy you have. The instructions will be clear on this. DO NOT deviate from its ratio. If it's 1:1, then mix 1:1 as best as you can. If you deviate then you'll end up with a sticky mess that will never cure. Read the instructions carefully before you use it.

... and verify if the ratio is by volume or weight and mix accordingly. I usually mix by weight. For epoxy that the manuf. specifies mix proportions by volume, I measure equal volumes of resin and hardener and weigh. I then have the proportion by weight. One common epoxy that I use which is specified to be mixed by "equal volume" is a 1:1.2 ratio by weight. I then just use my 0.01 gram resolution scale and mix accordingly.

dshmel said:

... and verify if the ratio is by volume or weight and mix accordingly

Click to expand...

Excellent point -- I've used epoxy that has different ratios depending on whether you're doing it by weight or volume.

-Kevin

Oh, I naively assumed that all epoxy was 50/50.

I am not suggesting deviating from the suggested ratio, so much as asking what would happen. More brittle, more gooey, or what. Also I have not been super precise with my 50/50 but probably hit within a percent or three.

What would happen would range from a gooey mess that will never cure, to a rubbery mess that would never harden. Except gooey mess is easier to clean off than rubbery mess. And all it takes is more than 5% deviation (in some cases) to do that.

The higher the resin to hardener ratio, the less forgiving they are to deviations.

taiwanluthiers said:

What would happen would range from a gooey mess that will never cure, to a rubbery mess that would never harden. Except gooey mess is easier to clean off than rubbery mess. And all it takes is more than 5% deviation (in some cases) to do that.

The higher the resin to hardener ratio, the less forgiving they are to deviations.

Click to expand...

Thanks! I assume hardener stays going while the resin will get rubbery?

No, the resin and hardener in epoxy are basically reactants in a chemical reaction, it is NOT a catalyst like polyester or polyurethane! The hardener is consumed along with the resin, the molecule becomes longer and longer then entangles to turn the resin thicker and thicker until it hardens. By using too little or too much hardener, the reaction is incomplete and therefore will be stuck in a limbo. Excess resin or hardener is trapped among the partially cured resin and of course you get a sticky mess. Only solution is to somehow get it off and redo it. Do not mix more hardener in gelled epoxy to make up for improper ratio, because you'll just waste more product and not get anywhere.

I see. I must have been mistaken about someone using only one or the other to treat a surface. Good to know about 5% tolerance as well.

Thanks much!

Some thin epoxy no more than 10% with denatured alcohol to "surface prep"tubes for fiberglass cloth application. This may be what you are thinking of.

Cardboard/paper tubes tend to soak up resin, by thinning it a bit the epoxy can be painted on with brush or roller very quick, where using the straight epoxy is gooey or so thick it can be very difficult to apply.

Of course all this depends on which epoxy brand & type you are using,
There are adhesives, laminating, filling, fairing type of epoxies to name a few.
All of these have different mix ratios [by weight or volume] 1-1 .....1-3....1-5.. 5.5-1..just to mention a few.

As always, just read the instructions before using and keep yourself out of trouble.


This is a good starting point that will answer just about everything you may wish to know about epoxy, including video how to and charts on what to use with what.
You buy all of their products at any West Marine store in any major city.
https://www.westsystem.com/ss/how-to-use/

Whether you use their product or not this is a comprehensive guide to any epoxy use, will keep you reading & learning for hours if not days.,

I don't know how many times I have heard someone say that to make your epoxy set up faster to use more hardener. It really doesn't work that way. If you want your fifteen minute epoxy to set up in five minutes......buy five minute epoxy. Honestly, anything quicker than that and you are not going to have time to place it where you want it anyway and your fifteen minute epoxy that is supposed to be mixed 1:1 when mixed 1:3 is not going to do what it was meant to do. This is chemistry we are talking about.

Probably an over simplification but carbon dioxide and carbon monoxide are the same things.....except for the ratios.

JoeG said:

This is chemistry we are talking about.

Probably an over simplification but carbon dioxide and carbon monoxide are the same things.....except for the ratios.

Click to expand...

Amen, brother.

Better living through science.

In Chemistry not only does a single atom make a HUGE difference (one is good for you, the other is a deadly poison), the way they are arranged makes a huge difference too! Isomer means 2 molecules, identical in every way except one is a mirror image of the other, and the two have completely different chemical properties.

By the way thinning epoxies aren't the right way to go about things. Epoxies you buy at hardware stores are designed for gluing, not composite work. That is not to say that it won't work, but it's not ideal. They are quite thick and the cured properties aren't optimized for lamination (they tend to cure somewhat like hard rubber rather than rock hard when used correctly). Laminating epoxies (they tend to have 3:1 or 5:1 ratios) are often thin, sometimes as thin or thinner than water. For example System Three are fairly thin, Aeropoxy is even thinner. Use the right epoxy for the right job, but if you must have a one size fit all system, get the thin laminating epoxy (West system and System three are all good) and thicken it with fumed silica if you want to glue stuff with it. Problem with laminating epoxy is the ratio makes eyeballing very hard since they are unforgiving in mix ratios. What I did with system three type epoxies when I mix in small amount is do it by weight. In my case weight and volume ratios seem to work out. Consult the instruction for your epoxy if you are not sure, some will tell you the proper weight ratio. a .01 gram scale will allow you to mix 1oz amounts for stuff like gluing.

By the way the epoxies designed for gluing tend to be optimized for a 1:1 ratio. This allows larger error in mixing, and also makes it easier to mix in smaller amounts.

Indeed. I recently tried to do a patch with some carbon fiber tape, using 15 minute epoxy glue instead of the laminating epoxy that I couldn't locate in my garage. The next day the patch flexed and fell apart. The epoxy, despite my attempts to saturate the tape, did not go all the way through, and left enough dry material inside.