Thickened epoxy using sawdust

How does one avoid hard bend points when building a boat? There are curves at the bow.

There will be bending of the sheer clamp at the bow, bending of 1/4" marine plywood sides at the bow (easier), and bending of 3/8" plywood bottom at the bow (harder). The hard bend points I referred to will be at the frames in the bow section where the epoxy could be squeezed out when the sheer clamp and plywood are bent to fit the curve.

 

Thanks for sharing your advice.

 

The only issue is if you are too quick with your wood mix. The individual wood grains in your sawdust must also be saturated. They will take in epoxy from the environment until they are. That might make a starved joint, even if you did everything else well.

Understand, this isn't a dire warning not to use sawdust in your goop. But you asked for possible issue. These are possible issues. If you'd asked me about possible issues relating to sleeping with Charlize Theron, I could provide some there too, but the overall advice would be "Go man go!"

 

if you trowel both surfaces being bonded with fumed silica and epoxy and a 1/16 vee; you'd have no issues..

the problem you will likely encounter is the sawdust may not flow well

to avoid overclamping; use less clamp pressure, or use a waxed screw in a predrilled hole; just avoid screwing it tight.. screws tend to not allow a tight margin, of course assumes a screw hole is okay

Last of your worries will be margin issues..

I sure hope it flows better than I expect it will..

 

How did you know that Charlize Theron is in the top two of my favorite actresses? .

 

Does pre saturating both sides to be joined negate concerns about thickened epoxy (WEST SYSTEM–#406 Colloidal Silica) being squeezed out at hard bend points? If so then there's no need to try using sawdust as spacers in joints.

 

Yes. If the feying surfaces are saturated, then epoxy squeezed out will not lead to starved joints.

 

Coloidal silica is a viscosity modifier, which is very different from adding sawdust. It takes a small percentage (in volume) of silica to increase viscosity. Sawdust will float in the epoxy and the mix will have low viscosity and run off the joint.

 

OK. Then I'll go with the pre-wet + colloidal silica method. As long as there's no concern about epoxy starved weak joints, then using colloidal silica will be easier than sifting sawdust.

Wish the write-ups and youtube videos that warn that too much pressure on joints will squeeze out the epoxy and result in weak epoxy joints, would point out that pre-wetting and using silica thickeners will negate that concern.

Thanks.

 

Pre wetting and silica greatly reduce but don't entirely eliminate risk of squeezing out too much resin with excessive clamping pressure.

Resin starvation can occur because:
1 Too little resin to start with
2 Resin absorbed by wood
3 Resin squeezed out by clamping pressure.

Wetting out and waiting until the wood has sufficient time to absorb what it will. Then applying thickened resin should solve the first two issues.
BUT even thick resin can be squeezed out if enough pressure is applied.

You should also strive to minimize the clamping force required.

 

The highlighted sentence in your response is the whole reason for my starting this thread. I don't see how the pressure between a fixed member (frame) and a member to be bent and screwed in place (sheer clamp, chine log, plywood side, plywood bottom) can be minimized without an excessive pre-bending exercise until the wood totally conforms to the curve required before attaching the member.

When I see folks building boats on youtube, there is a large amount of force applied to members to be bent where clamps or Spanish windlass are used to force the bending member against the bow stem. I mean a LOT of force. As a result, there's a LOT of pressure being applied to joints against the frames within the curve which cannot be minimized. In fact I remember reading (maybe a Glen-L article) that both sides can be installed symultaneously and brought together at the bow using a Spanish windlass to avoid twisting the frames out of alignment. This is why I was wondering if using sawdust with 1/16" (or so) particles would act as spacers to insure that the epoxy would not be squeezed out of the joints.

I don't see any way to minimize pressures at bend points in the curve.

Thanks.

 

From earlier in the thread

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.

I can add that multilpe cuts partially thru a piece of timber greatly enhances its flexibility

I once replaced all of the 3x3 ribs in an antique gaffer with laminated ones. Only tape was used to hold pieces in place until the epoxy cured.

Solid stock can be sized to be manually bent and held with little force.

Large sheets of plywood will require great force across their width.

Great force spread over a large area may not be excessive.

Some joints require glue and mechanical fasteners.


One can also take the precautions against resin starvation. Then immediately disassemble the wet joint and inspect for resin loss. Add a bit more resin with the reassembling.

 

I wish OP would pay attention to this post more. He is stuck on the concept that sawdust will maintain a margin. But what will happen is the epoxy will like to run out of the joint with sawdust. The mix ratio will need to be too high for decent wetout. Then the excess will be a runny friggin mess.

Clamp pressure of 9" of Hg results in ideal margins in silica/epoxy bonds. Say on 8" wide parts. Narrower, perhaps no.

4.42 psi !

So, for a 8" board, 2' wide, or say 200sqin; you could apply 800 pounds to it and no starve the joint. Let's reduce that to 25% for a 3/4" frame member and say 4' long. 36 square inches and 1.1psi is about 40 pounds of weight. The problem is rarely clamp pressure unless you use squeeze clamps capable of hundreds of psi typical..

 

What happens when you clamp epoxy/ silica joins is it shrinks. Clamp lightly to aim for 1mm margins and come back and all the clamps are loose. It is always unnerving for me, but happens everytime.

The bigger issues in clamping are resin drysuck from wood not precoated, poor substrate prep from epoxy cured for days and no mechanical key for secondary bonding; then overclamping. But it is rare. I've only had endgrain bond failures and probably was a process failure where the epoxy shrunk laterally in two directions away from the substrates. Sawdust sucks. The only time I used it, like I said, the ends of a canoe where I wanted it to be able to impact a tree or truck bed side. The stuff does not behave well in the mix..

 

I had good results with the dust I collected from my orbital sander.
I mixed it well and had no issues of glue starved joints.