Gluing With Epoxy: Proper Consistency

How thick can one mix epoxy (West System, FWIW) and still get good gluing characteristics? I've often read about "mixing filler to the consistency of peanut butter," but frankly that seems like an awful lot of non-glue material mixed in with my glue.

The reason I ask is, I'm trying to bond pieces of wood that are standing vertically, and an awful lot of the stuff seems to run out of the joint before curing. Is it really okay to put in so much colloidal silica and/or wood flour that the the goop is peanut-butter thick?

FWIW, I am painting both sides of the joint with uncut epoxy prior to gluing, and I am gluing in a workshop that is currently hovering around 75 degrees F and about 65% RH.

 

It'll still be stronger than the wood you are bonding, even peanut butter thick.

If you have a doubt, bond up a piece, then break it and see where it breaks. If it breaks inside the epoxy line, it needs to be stronger.

If it breaks the wood grain outside the epoxy bond line, you are good to go.

 

Folks often forget that the stuff they use to thicken epoxy is generally a form of reinforcing. Thickened resin is normally stronger than unthickened. Commercial epoxy putty is often so thick you have to cut it with a knife, yet it still sticks like crazy. Follow Catbuilder's advice,you'll not only get a better job you'll save a lot of money.

 

Actually, unthickened epoxy is usually stronger then thickened epoxy. This is mostly because the more stuff you add to epoxy, the more bubbles are introduced to the mixture, which weakens it a bit.

Harry is correct in that fillers added to epoxy are reinforcements, in that they add some physical attributes to the cured matrix.

As far as viscosity and consistency are concerned, this is generally application specific, as well as environmentally dependent. Hot days need more fillers then cool days, vertical structural fillets need much more filler then flat work, etc. As a rule you add what you need for the task at hand.

If making a vertical "glue" then use the filler you need, such as cotton flock, milled fibers, etc., then add just enough silica to stiffen the mixture enough so it will "stand" when a putty knife is pulled out of the mixture. If the little lump begins to fall back down, it will sag in the vertical joint as well. It's a feel thing you'll develop as you get some experience with goo.

 

And... that's about the worst thing that can happen. When that happens, you have some very difficult sanding ahead (and more filling), or you have to keep on going and make sure you fill in the sag before it cures.

When I was starting out, I remember having lots of sags in the cracks between core I was bonding. It took a very long time to sand those little cracks, then fill again when I should have been done in one pass.

2nd hull is always easier than the first!

 

You didn't mention whether this is structural or just interior/cabinet wood work. Just keep in mind that a good waterproof wood glue will glue wood to wood as strong as epoxy. I use it for interior work if it is not structural. It is easier, faster and cheaper to work with.

CAt, as your wife would say, sometimes the best glue is not an Epoxy... lol

 

Planks to chine is the current application, which I'm putting on the "structural" side of the ledger.

 

Don't forget the two stage technique - i.e. thin coat both the faces for penetration, then thicken and apply to one face, and then join. There's a definite limit to how thick you can make it and expect it to penetrate and bond to the wood surface.

 

I've just started using West System. I find a 7g measure of #403 microfibers per 1 pump measure of epoxy & hardener provides a non-sag mix that still wets the surface adequately; it was a bit think for laminating so I cut to to 5g which seems fine. The 5:1 mix seems stronger than the 1:1 formulation I was using before but I've only done one test piece so far.