Thickened epoxy using sawdust

There are several issues with using sawdust. The first is contamination, you don't really know what is in it. Second, it is an absorbent load. That means that, as was noted before, it may starve the joint. The only way to keep the joint wet, is to make the mix runny, which does not work well for fillets. Third, as has been noted by other posters, the finish will be rough and hard to finish. Fourth, it will use more resin than fumed silica, which will offset any savings. Epoxy is a lot more expensive than silica. Finally, the rough surface will greatly increase the labor and the cost. You will have to use abrasives and cleaners.

 

So, it's a seductive, romantic, fantasy.
You, once again, raise good points Gonzo.

 

I figure that the 1/16" sawdust particles will be compressed somewhat plus indented into the wood to be bonded. Maybe ending up as 1/32" inch.

At any rate, since there has not been any confirmation of my plan, I'll do some test glue ups on a corner (2.5" x 2.5") of two pieces of 3 feet long 1x3's. Another with 1.5" x 1.5" glue surface and a third at 1" x 1". Will screw the glued joints together so that the screw heads are counter sunk. After 3 days I'll remove the screws and then do sheer and pulling apart tests. If the epoxy breaks, will discard my idea. If the wood breaks I'll go ahead with my idea.

 

See my post #18. Thanks.

 

The sawdust will actually swell causing the joint to be more than a 1/16".

I would aim at a glue line of 1/64".

 

The ideal epoxy wood to wood face bondline is 40 mil or 0.040 per some reading I've done, sorry; the citation eludes me at the moment. But 1/16 is not far off. The trouble is you can't trowel it at all to 1/16". You'd be lucky to get 1/16 sawdust to trowel to 1/4" well and the bondline would end up about 1/8" which would use an awful lot of epoxy.

I am not a fan of sawdust for wood to wood bonds. I have used it where I wanted to strengthen a join in a canoe at the ends of the canoe like a big giant fillet, but honestly fiberglass tabbing would have been lighter and less likely to allow ingress.

You'll find out for yourself that it is nearly impossible to trowel.

An ideal epoxy joint would use fumed silica and gets mixed about 2.1-2.2:1 to epoxy. Then trowel on each side with 1/16" vee trowels and compress using about 9" of mercury with vaccum. These joints are about the ideal size. It is inportant to prewet all epoxy joins because the substrate can absorb the resins leaving dry fillers behind. Doing this is easiest of you let the epoxy cure to sticky before troweling on the thickened resins.

 

Large pieces of sawdust (1/16") won't easily thicken the epoxy so that it won't sag, the finer wood flower and/or silica is what will thicken it so it doesn't sag or flow out of the joint.

The 1/16 pieces can be used as spacers in the same way as beads are used in some adhesives to achieve the correct gap.

To achieve this you need a large amount of silica or wood flour and a relatively small amount of sawdust.

 

an excellent point I forgot; the epoxy will like to sag out of the wood sawdust bits...
The 2.1-2.2 mix I cited has to be thick enough to not collapse as well..

 

I already bought the West Systems notched plastic spreader. It has 3 sizes of saw tooths on 3 sides. Hopefully it will work out with the sawdust. My bonding strength tests will let me know.

 

The problem I have had is the thicker bits and variations in particle size tend to make flow difficult. I doubt you'll be successful troweling it on evenly.

 

One other question. It's said to control clamping pressure to not squeeze too much thickened epoxy out of the glue joints resulting in weak epoxy starved joints.

How does one avoid squeezing standard thickened epoxy out of the joints at the critical bend points when doing hard bends of chine logs, sheer clamps, side and bottom plywood in the bow area? Seems to me that there is no way to reduce the pressure at the hard bend points when applying the force needed to secure the items to the bow stem post or in the case of jon boats, to the horizontal prow member.

Just attach the items and let what ever squeezes out at the hard bend points be epoxy starved?

 

Pre coat the affected areas, so that they are saturated before you close the joint.

 

You should not have hard points. Design the layup so that you avoid hardpoints. What are you gluing besides two flat boards?

 

Yes yes yes
Let the pre -coating flash off before bending into place.
Dry clamp and over bend the boards and leave in place for a few days prior to gluing.
Use thinner lumber that requires less effort to flex. Multiple stacked slatts
Csm in the glue joint.

 

If I understand your post, if joints are pre-coated (saturated) with unthickened epoxy before joining, then there is no concern about epoxy starved joints, regardless of pressures involved like clamping, screwing or bend points? ... I did plan to pre-coat all joints (frames, chine log, sheer clamp) regardless of whether joints are flat or at bend points.