Some Epoxy Handling Tips #2: Mixing and Additives

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Blog entry by shipwright posted 11-10-2011 08:23 PM 2640 reads 6 times favorited 13 comments Add to Favorites Watch
« Part 1: Don't get any on you ! Part 2 of Some Epoxy Handling Tips series Part 3: A Little Trick with Cloth »

When I posted the first blog entry here some questions about mixing and additives arose. I find that often useful answers to good questions get lost in a list of comments when they could be much more easily found in a separate post so here is that separate post.

Disclaimer: Although I have had extensive experience in the use and handling of epoxies, I have not tried all the various brands and manufacturers’ products. My formulator of choice was Industrial Formulators of Canada for the following reasons.
1) The company was owned and operated by the chemist who designed the formulations and he was always available to answer questions, a brilliant man.
2) For the purposes that I needed epoxy (boat building), they had the best line of products based on actual chemical characteristics. IFC has been absorbed by System Three who now carry the old IFC products.

There are lots of manufacturers and formulators and each has its fans. I do not want to get into an argument about who is better than who. The information here is pretty generic and should apply to epoxies in general.

Mixing Epoxy:

When you mix epoxy you are combining two chemical reagents that join together to form a new molecule. The molecule is formed at a fixed ratio that may be 1:1, 2:1 or some other. The important thing is that as cross-linking takes place and the new polymer molecules form, the reagents are used up at an exact ratio. When the last molecule is cross-linked the reaction is over and any un-linked reagents remaining in the mix will have a detrimental effect on the properties of the polymer. This is why it is important to mix as accurately as possible.

Perhaps because of the widespread use of polyester products like fiberglassing resin, Bondo and such there exists a belief that adding more or less hardener is an acceptable way to hasten or slow the cure. This works for polyester because it is a catalyst reaction. It is not a good idea with epoxy. You may change the cure rate a little by increasing or decreasing the number and proximity of available partner ions, but when the reaction is over you will have remaining un-linked chemicals in the mix. They may affect water resistance, strength, hardness, etc.

My personal preference for getting an accurate mix is to use graduated cups or tubs. I have used the metered hand pumps but find them unreliable when the materials are cold particularly if one is thicker than the other at normal temperatures.

Blend Completely

Follow the instructions. The thicker the components, the more important it is that you mix thoroughly and scrape the sides of the container frequently like the label says. The glue stick that I described in the first blog entry has one side left straight for this purpose. For me , hand mixing affords a better feel for the mix and allows me to make sure that all the reagents are involved.

Blend the clear epoxy fully as per instructions before adding any additives. Removing poorly mixed goo from a large area can be a real pain…...don’t ask.


There are two general kinds of additives. One is absorbed by the epoxy and makes little difference to the volume when added and the other is not absorbed and increases the volume by however much you add.

Fillers are the non-absorbed additives and are used to make epoxy go a little farther or to give better sanding and gap filling qualities. These are very handy in fairing slightly uneven surfaces like boat hulls or the pattern left on the surface after sanding a fiberglass cloth layup. They are mostly various sizes of micro glass spheres and the make sanding much easier.

Thickeners are the additives that are absorbed by the epoxy and they can greatly increase it’s ability to stay where you put it. Epoxies in general heat up as they cure and become less viscous. The right additive (my favorite is called mini-fibers) can dramatically reduce the “sag” or eliminate it completely without compromising the strength of the joint. Fillets made with mini-fibers are very strong and can be used as structural joint. I often mix some mini-fibers for thickening and some micro-spheres to aid sanding in the same mix. There are all sorts of properties you can coax out of one epoxy with the right additives.

Well, I didn’t mean for this to be this long or this dry, but I can’t really find anything that I want to cut so I’ll be kind and stop here.

I hope it hasn’t been too thick and that this will help some of you better understand epoxy.

If anyone has specific questions that I can help with, please let me know.

Thanks for reading. Questions, comments and critiques are always welcome.

-- Paul M ..............If God wanted us to have fiberglass boats he would have given us fibreglass trees.

13 comments so far

View Dez's profile


1166 posts in 4102 days

#1 posted 11-10-2011 08:31 PM

As always I find this post very informative!
Thanks for sharing your knowledge!

-- Folly ever comes cloaked in opportunity!

View Lee A. Jesberger's profile

Lee A. Jesberger

6859 posts in 4004 days

#2 posted 11-10-2011 08:37 PM

Very well written, Paul.


-- by Lee A. Jesberger

View Schwieb's profile


1858 posts in 3486 days

#3 posted 11-10-2011 09:26 PM

Well done Paul. I had to learn some of these things by reading and trial and ERROR when I was glassing the kayak. Wish I had you watching over my shoulder when I was doing that. My son is building one also and doing the second will be much easier. Thanks for sharing your wealth of knowledge.

-- Dr. Ken, Florida - Durch harte arbeit werden Träume wahr.

View BritBoxmaker's profile


4611 posts in 3061 days

#4 posted 11-10-2011 10:27 PM

Well written and informative, thanks, Paul.

-- Martyn -- Boxologist, Pattern Juggler and Candyman of the visually challenging.

View chrisstef's profile


17424 posts in 3031 days

#5 posted 11-10-2011 11:25 PM

This is great information. I will be venturing into my first bout with epoxy shortly and have read this with great interest. Keep up the info im eatin it up. Im sure ill be messaging you about my venture into the epoxy world. Thanks Paul.

-- Its not a crack, its a casting imperfection.

View Roger's profile


20928 posts in 2829 days

#6 posted 11-11-2011 01:22 AM

info like this is always a gr8 help. thnx for the education Paul. more info is alway good info

-- Roger from KY. Work/Play/Travel Safe. Keep your dust collector fed.

View Napaman's profile


5526 posts in 4102 days

#7 posted 11-11-2011 07:14 AM

loving this…

New Question: Micro-balloons…I see this mentioned on boat blogs and articles…what is this? Any connection?

-- Matt--Proud LJ since 2007

View shipwright's profile


7992 posts in 2823 days

#8 posted 11-11-2011 07:41 AM

Matt, micro balloons, micro spheres, Profil, likely lots of other names are all tiny hollow glass spheres. They come in coarser and finer sizes but they are all microscope stuff. The one I use is Profil because it’s the one my supplier handles.

-- Paul M ..............If God wanted us to have fiberglass boats he would have given us fibreglass trees.

View Napaman's profile


5526 posts in 4102 days

#9 posted 11-11-2011 04:22 PM


Are they used as a filler? Floatation? How does that work if so? Would you use it with all glue ups so you have floatation added? Or is this not at all the purpose…

-- Matt--Proud LJ since 2007

View rance's profile


4258 posts in 3185 days

#10 posted 11-13-2011 11:58 PM

Interesting and useful blog Paul. I am also interested in different fillers to add color. I have acquired some toner from several used laser cartridges I plan on testing. I’ve used colored sawdust in the past that worked good. I’ve also seen folks using one of the K&B pigment products(25+ years ago). I’ve also seen glitter used, but I’m not a fan of it. What other ‘creative’ compounds have you used or do you recommend(or not recommend)?

-- Backer boards, stop blocks, build oversized, and never buy a hand plane--

View shipwright's profile


7992 posts in 2823 days

#11 posted 11-14-2011 01:46 AM

I’m not really qualified to talk about pigments that are compatible but I know that there are compatible pigments. Ask your supplier or manufacturer.
I’ve mostly only used black in the form of either carbon powder or in a paste that I got from my epoxy supplier and white in the same paste form. I used them between teak overlay strips on decks (black) and cabin soles (white).

-- Paul M ..............If God wanted us to have fiberglass boats he would have given us fibreglass trees.

View Napaman's profile


5526 posts in 4102 days

#12 posted 11-23-2011 02:50 AM


I have been using wood flour so far exclusively as a thickener…but I ran out…so I finally used silica…

Not sure why but I was nervous for some reason…it seemed to need more mixed in to thicken up but seemed to work fine…it did seem to take more mixing to get the bunches mixed in—-like the pancake mix that seems to have clumps of dry/unmixed content with each stir…

How will each sand? Any think I should know using the silica? I was wondering about safety—-I assume a dust mask minimum…I have a mask with dual respirators…

I DID go back and scrape excess drips after words but with the boat flipped I could not get the underside…


-- Matt--Proud LJ since 2007

View shipwright's profile


7992 posts in 2823 days

#13 posted 11-23-2011 03:09 AM

Matt, If you are using a good thickener, there should be no drips to clean up. Your epoxy should be a putty, not so dry it won’t wet out the surfaces, but not so wet it sags. A good test is to scoop some out on a stir stick
so that it is about a half inch thick. Hold the stick up and the epoxy should just stay on the stick, a half inch thick and not sag at all. With mini fibers you can get that to about an inch or more without making it too dry.

I’ve never used silica but it sounds like a filler to me. What are you trying to accomplish by adding it?
Maybe there’s something better to use for the purpose.

-- Paul M ..............If God wanted us to have fiberglass boats he would have given us fibreglass trees.

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