Thread: Dry Scarf Joints

Hi Jethro, do you have a picture or two of the bad joint?

Ian

No sorry Ian,

I can see that each face of the scarf is shiny and covered in clear epoxy, whereas the rougher marks have the white colouring of the glue mix.

I'm now tossing up whether to finish off with use butt joints, fibreglass joints or a bailing bucket.

Regards
Geoff

Originally Posted by JethroT

...or a bailing bucket.

I'm not sure what your solution will be, but clearly you've got the right attitude! Good luck...

When mixing epoxy I use translucent drinking cups. After mixing the epoxy I add silica, while looking through the side of the cup, to a point that doubles the depth. If there's an inch of epoxy in the cup I sprinkle an inch of silica on top of it. That gets close to the consistency I want then I add a bit more if needed.

I wouldn't use thickened epoxy in a joint without saturating the wood with the thin stuff first.

Your description sounds like a starved joint. Some things that can cause that are not saturating the wood well with thin epoxy before applying the thickened stuff, too much clamping pressure, not enough glue, glue running out of the joint, and squeezing the glue out of the joint then easing clamping pressure.

Is this bad scarf joint joining ply sheets, spars, chine logs, gunwales or what? Is it possible to saw it apart and re-do it? I think that would be the best option if at all possible.

Ian

I think mixing thickeners by volume will be too difficult. Some say peanut butter consistency, but not all peanut butters are the same. A good mixture is one that will peak and then slump down slowly. If it holds a peak, then that is too thick, but is right for filleting.

I had the same thing happen on my original boom, which broke. On close inspect, the break started when the scarf joint failed. It is important to coat both surfaces of a scarf joint with unthickened epoxy first, and then apply plenty of the thickened epoxy over that. Too much pressure causes a dry joint, so that is likely what has happened to your joint.

Instead of pulling it apart, is it practical for you to add a butt strap?

Thanks guys,

I was joining two 8" sheets of ply, and will try again with some new found knowledge.

Originally I coated each surface with thin epoxy, then mixed the glue, however from your replies it would appear that I made the glue too thin and from memory I put a lot of initial pressure on that edge and then held it down with less pressure.

Now back to the shed.

Regards
Geoff

Back to the shed,

This time I mixed up a much thicker glue and it didn't self level as much as the last mix so hopefully things will work out fine this time.

I'll let you know tomorrow.

Regards
Geoff

With epoxy, you only need enough clamping pressure to bring the two pieces in contact with the goo, that's it.

Wetting out raw wood prevents the dry wood from sucking out the resin and leaving you with only filler powders in the joint. A simple wet out and wait is the usual approach. If the raw wood looks like it might want more to "drink" apply some more, but in most cases the initial wet out is sufficient, before applying a thickened coat.

The viscosity of thickened epoxy is dependent on many things. The rule is, no thicker then you need. On vertical and over head surfaces, you'll need a non-sagging mix, which is a heavy peanut butter consistency. The pulled up peaks remain as you mix. Most adhesive mixtures don't need to be this thick and a heavy cream consistency is all you need. This will "slump and sag" when you pull up a peak during mixing, but inside a joint will remain in place from the friction of the joining pieces.

How much, well it depends on temperature, humidity, your in laws mind set and the color of your hair. There are so many variables, that precise mixtures can't be offered, except in environmentally controlled conditions.

As I often find myself saying ... you guys are really good!

MIK