Scarf error, microballoons used as epoxy thickener

I used microballoons by mistake to thicken the epoxy used in the scarf joints and I am concerned about the strength of the joint. (I thought microballoons and cab-o-sil were the same thing.) 

I have now read that microballoons are not to be used in structural joints. What should I do? Should I be safe and cut out the scarfs and rescarf? Or will the joint be OK. Maybe I can strengthen it another way.

I used about 50% epoxy to 50% microballoons by volume in the mix. 



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RE: Scarf error, microballoons used as epoxy thickener

I would absolutely take the joint apart and reglue. I can't speak for the strength of epoxy thickened with microballoons vs cab-o-sil but for peace of mind I'd want to do it right.  If you don't you will always be wondering if that joint is going to fail.

If you are plan building just cut the scarfed area, rescarf the panels and away you go. If you are kit building, take a hot iron and heat up the joint until you can pull it apart. Scrape off as much epoxy as you can, applying heat with the iron as needed. Smooth the mating surfaces and Bob's your uncle.


RE: Scarf error, microballoons used as epoxy thickener


Since it's a slow, light kayak (not a large hish speed performance boat) a glass patch over each scarf line will also take care of the problem. Make the patches between 10 and 20 cm (4 and 8 inches)  wide, centered on the scarf line. The bigger the patch, the stronger the joint, up to a point. Cut the glass on the bias so that the threads cross the line at a 45 degree angle for maximum strength. For a simple kayak, 4 oz woven cloth should be fine, 6 oz will make it bulletproof (but at the cost of a stiffer joint).

The microballoon mix will handle the compressive forces just fine (the okoume will crush long before the microballloon/epoxy mix). The glass will handle the tension, torsion and shear forces and limit those forces to what the balloon mix can handle. The presence of the deck, bottom and side panels will also limit the torsion to a safe level, so you won't have to worry about your boat breaking up around you.

Depending on the type of boat, the extra glass could cause a bump when bending the panels. In that case, re-scarfing the way Dusty describes could be a better choice.

Good luck,


RE: Scarf error, microballoons used as epoxy thickener

If the epoxy is still green or softer, pulling them apart won't be much of a problem and would be the way to go.  If the epoxy has set, I think you will tear up the hull panels if you try to take apart the scarf joints.  The hull should go together just fine with your panels as they are by you being careful handling them (i.e., don't try to break the scarf joints when you move the hull panels).  The glassing on the outside of the hull will strengthen the joints and you can add more glass on the inside to soothe rupture fears.  Your cockpit area should be glassed anyway (many S&G have their scarf joints in that area).  Once you have the outside and the cockpit glassed, your hull will be strong.  That would be a good time to add additional glass on the inside of each scarf joint for bulletproof joints.  However, if you follow Laszlo's recommendations, be sure to keep any glass (including loose glass threads) away from the area where sheer clamps attach and leave a space on each hull-panel edge where the panels join.  And lightly sand/scrape each patch before wiring the hull together.   Also, to minimize the possible 'bump' in the panels if you use glass patches on the inside,  use oval- or diamond-shaped patches with the pointy ends extending along the long axis of the panels.


Good Luck.  

RE: Scarf error, microballoons used as epoxy thickener

Thanks Dusty, lazlo and ootdb for the good advice.

I am a bit reluctant to cut out the scarfs and rescarf because the panels will then be too short and will require another scarf to bring them up to the full length required. This translates to 12 scarf joints which is a lot of work - not so much cutting the scarfs, which I have become proficient at, but I find aligning them and glueing them to be time consuming.

I like the extra glass on the inside idea. But I would like to hear further comment particularly from the staff at CLC before I commit to a solution.

I am not concerned about the finish as this is my first boat so a bump here and there is not a problem.





RE: Scarf error, microballoons used as epoxy thickener

Alain - I'm assuming that you have the flat hull panels not yet assembled into the hull shape, that is, they are still flat.   I think you'd find that if you assemble the panels into the finished shape with the stitching, and THEN apply the extra fibreglass 'patches' inside the hull, over the scarfs, they will naturally follow whatever modest curve is now induced into the hull (sides and bottoms) and no bump will be evident, even to the most critical and microscopic study!   I'm ultra-fussy, and it's what I'd do.   As a matter of routine I'll probably be putting a 3" tape+epoxy strip over my scarfs inside the hull when the panels are stitched together.   And as someone noted above, you'll have the cloth+epoxy skin on the outside making the cloth-epoxy/ply/cloth-epoxy laminate pretty much bullet-proof and flat as a tack......

Hope this gives food for thought and a means of avoiding all that extra work...  Lol from Oz

RE: Scarf error, microballoons used as epoxy thickener

Yes I am at the flat panel stage building from plans (no kit). I have the panels scarfed and the shapes laid out ready for cutting.

I am going to do the extra tape on the inside of the joint after stitching route rather than rescarfing.  (Unless someone at CLC says it is absolultely not the way to go).

Thanks again.

RE: Scarf error, microballoons used as epoxy thickener

Hey guys,

My post seems to have been misinterpreted a bit. The extra fiberglass patches, if you use the method I described, are meant to go on the outside. That's where the joint has the greatest tension, which is what the balloon mix is the weakest in. The glass patches are also meant to be in addition to the glass that's normally applied to the outside of the hull.

If you're going to skip additional glass, the place to skip it is on the inside, where the forces are compressive. Balloon mix is stronger than okoume in compression, so the wood will fail first anyway.

Also, shaping and shortening the patches is not optimum. For maximum strength, the bias-cut glass should run the complete length of the seam and it should extend 2 to 4 inches on each side of the seam. There is no need to leave room for the sheer clamps because the epoxy glue will easily fill the slight gap caused by the extra thickness of the glass.

What I described is converting the scarf joint into a true composite joint. It is structurally sound for the small kayak application and very conservative engineering practice with lots of margin for error. Shaping, reducing, skipping sides, etc. take away from the ideal structural composite chracteristics, the same way that using balloons takes away from the ideal scarf joint characteristics.

As the builders, we are of course free to do whatever we want. I just wanted to make sure that no one misunderstood what I was recommending.

Happy building,



RE: Scarf error, microballoons used as epoxy thickener

Second sentence should read:

" The extra fiberglass patches, if you use the method I described, are meant to go on the outside, too"

Wish there was an edit facility for posts. Othe forums have them.



RE: Scarf error, microballoons used as epoxy thickener


Since you are building from plans you have some scrap ply left over.  Make a few test scarf joints using the same epoxy-goop mix you used when epoxying together your hull panels.  Let them cure properly.  Then break them to see how fragile your joints will be.  Also, break a few scrap pieces the same size as your test panels to have a rough breaking-strength comparison.  If your original epoxy-goop mix wasn't starved by too much epoxy being wicked into the wood, you'll probably have reasonably strong joints.  If you make the test joints using the same method you used for the scarfed panel joint (don't cheat and try to make better joints), and the test panels break cleanly at the scarf surfaces, you'll need to add glass patches.  If the test panels break in the ply, you don't have a problem.  If the test breaks are a mix of wood and scarf surfaces, I'd add glass patches.


If you do run a test, please, please, please tell us the outcome.


More food for thought:  many of the new CLC kits are built using puzzle joints with glass patches on the inside.  ...


Good luck

RE: Scarf error, microballoons used as epoxy thickener

Hi Lazlo,

 Thanks for the clarification, that makes a lot of sense. I will put 6 inch wide additional patches of glass on the outside of the scarf joints (where the tension is).

ootb - yes I have a few pieces left over which I will try test. The original joints are well cured so the tests would fail sooner.

Another thing - I made the scarf overlap by 1 and half inches rather that the recommended 1 and a quarter inches for 4 mm plywood (also by mistake). I guess this will make the joint stronger.

I will need a strong boat - I will be paddling it in Cape Town (The Cape of Storms) where we regularly paddle surfskis in gale force winds. See

Not that I would take a Mill Creek 16.5 in anything like that.


RE: Scarf error, microballoons used as epoxy thickener

The failure mode in bending of a proper scarf joint with no balloon is the plywood breaking.

If you have a chance to test on scrap wood, as ootdb suggested, the results would be interesting.  If the failure mode is no different, then scarf joints made with the balloons are fine, with none of the reinforcements suggested above being needed.

RE: Scarf error, microballoons used as epoxy thickener

ootdb wrote "More food for thought:  many of the new CLC kits are built using puzzle joints with glass patches on the inside.  ..."


You are absolutely right, my WD12 has joints like that. In that case the glass is there to reinforce the joints until the boat is actually stitched together. Once the boat is assembled, those patches do very little, if anything, in terms of structural support.

BTW, your idea to actually test the joints to breaking strength is really the only way to  get exact answers. Even better would be to test the boat itself, since the panels all support each other. Wonder if we can get some finiancial & material support from a sponsor to set up a boatbuilders test lab?

Alain, 4 inches should be plenty wide.

Have fun, all.



RE: Scarf error, microballoons used as epoxy thickener

I tested the joint on a scarf that I had cut out from a previous blunder. It was just an inch and a half of scarf joint so I could not test it properly. But when I tried to pry it apart using a screw driver the wood delaminated before the expoxy.

 I am busy glueing up some test pieces right now which I will test tomorrow. It looks like the microballoon mix is strong enough.

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