ContourKore Endgrain Balsa Laminates?

Anybody ever post this up for review before?

Dsiclaimer: all info provided here without permission of author…

I happened to venture off there on a whim and found a very interesting quote…


Once all the balsa sheets are cut, they are coded for position, stacked (scrim side down) and a coat of resin is rolled on them as thinly and evenly as possible. This “hot coat” prevents the wet laminate resin from penetrating too far into the balsa during vacuum bagging. A gel time of 15-20 minutes is used to attain as rapid a cure time as possible; ideally, the balsa is left overnight to cure completely

These guys vacuum 1oz fiberglass mat to the outside of this ContourKore endgrain balsa laminate and then sealed with layers of glass over that…

Here’s smoe other interesting quotes:


The Olson 30 is built with a balsa-cored hull and deck. Though balsa is a sometimes ‘maligned’ core material, it is
in my estimation the best, considering the strength-to-weight properties.


"Why not foams? We have tested synthetic foams extensively, but never put them in a boat. We found, in testing, that under point

compression loading, foams crack and fracture the skin on one side, as compared with the balsa core. A fracture might go all the way
through a balsa/FRP sandwich, but it takes a lot more force to do so than with foam. The main difference is that if a balsa core is
damaged, you can pinpoint the area. Synthetic foam damage, on the other hand, is extremely difficult to pinpoint. On one side of the
laminate you may see a fractured gel coat, but on the other side it could look perfect. It would look like the simplest thing in the
world to repair, but when you look at it head-on (cross-section), you will see that the foam was totally sheared into two pieces.
"A balsa/FRP sandwich has amazing strength. We did impact tests on both foam and balsa core, with the balsa maintaining nearly
100% of its structural integrity at the point when structural failure was resulting in the foam.

Hot coating and sealing the Balsa prior to lamming the glass to it hmmm…

Is this Bert’s secret i,e, building a Balsa/FRP sandwich under heated vacuum but basting the wood first to reduce the resin intake?

Anyway thought it was interesting as I’ve never heard this one before.

Everyone’s been talking about basting your glass job or spackling the foam closed… No ones talked about sealing the wood before applying the glass…

Comments from the iso-peanut gallery?

Is this Bert’s secret i,e, building a Balsa/FRP sandwich under heated vacuum but basting the wood first to reduce the resin intake?

i’ve tried lots of things, including the above, to get balsa to soak in thin resin…haven’t been able to do it…similar to pressure treating outdoor lumber (autoclave; heat vacuum then pressure)…benny would proly know more…but it just aint happening in my garage…

he’s got lots of secrets but im betting this specifically is not one of them…im betting on others…

the other things you mentioned are cool though…i’ve been waiting for others to post similar thoughts

here’s a simple test…take some really thin resin add coloring in it so you can see penetration…place a drop on a pre-heated piece of balsa let that soak…try the same with a vacuum pull…after its dry cut the wetted spot(s) with a blade and check for depth of penetration…

funny thing is a balsa tree is 70% water when its live but after kiln drying the stuff doesnt soak…there are epoxy products for just that purpose…penetrating epoxies for wood…theyre just solvent thinnned proly with xylene…balsa wood has surprising properties…so in short pre-coating the surface to prevent further absorption may not be the best way to spend time…but hey thats just me

I’m working on an all d-cell sandwich board, like in Bert’s first thread. Your questions go along towards what I’m finding with that.

My core is 1# EPS. I bagged on the bottom skin last night and tried something new. First, I shaped & countoured the blank but didn’t cut the template. Left it 10’ long and 2’ wide. Planned on cutting 4 oz glass cloth & 1/8" d-cell also to 10’ x 2’. I’ve also wired out the deck rocker and I’m using that deck offcut as a rocker table, to go in the bag with the blank.

I sat there looking at the EPS core and, after vacuuming out all the planer & sanding block dust, it looked damn porous. So I decided, for the first time, to try sealing. I didn’t want to wait for spackle to dry and I have to admit I’m hesitant to put water-soluble stuff inside a surfboard. So I mixed up 16 oz of epoxy and added cabosil until it was about like mayonnaise. I squeegeed it on and only ended up using about 12 oz, until the blank was smooth & shiny. It was 77* in my garage.

About 3 hours later, the skim coat was only slightly tacky. hard enough that If I laid on glass cloth, I could still smooth out the wrinkles. So I cut my cloth & mixed 16 more oz of epoxy. I laid on the cloth and poured out the resin. After wetting out all the cloth - keep in mind this thing was still a full rectangle, 10’ long - I scraped about 4 oz back into my bucket. The sealing, especially with a thickener, sealed the EPS so well my resin use was waaaaaay down. Normal glass-onto 1# EPS-epoxy lamination for a 10’er is around 32 oz of resin - and that’s a templated board with the corners cut off. I used, between sealer & laminating resin, less than 24 oz. So I cut my resin use by more than a third. More, when you consider what is still to come off. Then slap on the d-cell & the whole rectangle, with rocker table, went into the bag.

5 am this morning, I couldn’t sleep any more. It’d been about 9 hours in the bag. I go outside, shut down vacuum, and take it out of the bag. Beautiful, smooth, perfect skin lamination. So I template the shape & cut off the excess with the jigsaw. That means I have pieces to test :slight_smile:

So I started breaking stuff apart. The d-cell & glass would NOT separate from EPS, no matter how I tried to peel it, snap it, bend it slowly, whatever. Every time, the EPS failed a few mm away from the d-cell - probably where the sealer had soaked in to.

So, long story short, yeah, Oneula. Sealing. I don’t know if you even need to seal the balsa, just seal everything else & slap the balsa on last. Use so little resin that there’s no excess to soak into the balsa (like I had so little that there was no excess to soak into the d-cell) and its not an issue. But in terms of keeping weight down & reducing resin cost & acheiving a better resin/cloth ratio? I’m sold.

Took me long enough, eh Greg? :slight_smile:


I don’t think that end grain balsa has a place in surfboards, it has great compressive strength, hence its use in yacht decks, but does soak up the resin, paticularly through the gaps between the blocks which make it good for resin infusion though.

Dale Solomonson has referred to it’s use in flex spoons…;search_string=end%20grain%20balsa;guest=2032047#148062

I forget who incorporated the end grain balsa transition in this flex tail (third from right.) The board was brought to S.A. 2003 by either Kirk Putnam or Matt Miller… maybe one of them can fill us in?

Didn’t Platty recently build an entire board out of end grain scraps he collected?

I notices that Fiberglass Supply where I saw this sells it as a flexible scrim backed laminate for use in some type of board making. 24"x96" sheets.

with the grain running perpendicular to the load the compressive strength must be unreal maybe over a 1/8-1/16 layer of divinylcel/corecell as you could eliminate the compressive breakdown of the foam core the skins are surrounding which is the inherent problem of all boards in general. Almost like that Nomex honeycomb stuff ACP sells. No Bert spingboard though…

Interesting that you can seal the end grains but not the sides like what Meecrafty aludes to seems like the ends would suck resin more…


I DAP’d my last Lowes EPS fish as test no wood rails but I think I’ll do a vent on mine.

This new Subway sandwich special is as follows:

Core = 1"Foam+Bamboo+2"Foam DAP “Quick&Final” Spackle sealed

Skin = epoxy+4oz E+epoxy+1/16 Balsa+epoxy+1/32 Birch/Mahogany/Bamboo+epoxy+4ozE+epoxy

We’ll see but I agree, sealing is a good idea as well as basting the lam.

I might try that Qcell-Epoxy paste but its easier to sand DAP that epoxy to clear out any ridges…

You ever get that bamboo sample?

What’d you think?

One, you can get the data for the stuff here.

The board that I built, was built with end grain balsa. But I glued and milled the logs so the grain was running parallel to the stringer.

I have used EGB extensively in boatbuilding as a core material. We always hotcoat the balsa prior to applying it to the laminate. The cloth is wetout and rolled and left to gell. Once gelled a bonding material is applied to the laminate. Normally resin and aerosil mixed to a stiff consistency. The balsa is coated with a hot coat(unwaxed) and pressed into the bog. This is a pretty basic way of bonding.

If a vacuum bag is going to be used the balsa is either layed on a wet laminate or the above system is used. Caution needs to be taken when vacuuming balsa on a wet laminate. To much suction and all the resin will be pulled through the balsa.

Some balsa suppliers pre coat the balsa. platty

We always hotcoat the balsa prior to applying it to the laminate

hey platty,

for what purpose?

what is the goal of hotcoating the balsa?


I don’t think that end grain balsa has a place in surfboards… …but does soak up the resin, paticularly through the gaps between the blocks…

  Why not?      I've been thinking about trying it myself, it seems easier to work with.  The boat builders are doing it for a reason.  You might not get the twang but you'd really have to run a test to compare the two to actually know.  I don't see why you'd have any bigger problem with resin collection between sheets of end grain than you would with wood oriented in any other direction, it all boils down to how clean you  make the two peices meet.  I can however see how the end grain would suck resin, so sealing it makes perfect sense to me.  A thickened quickset seems like the best bet for this.     Thoughts on a twang testing apparatus.  If you had two test sandwhiches with equal dimensions, about 3' long, clamp them to a table side by side with most of the material hanging over, you could pull down and let them spring back, see which comes back quicker and with less memory.  If they were real close you could set up a back stop to see which hit it first.  You could pull down with a scale (like for weighing fish by the lip) to compare stiffness.

End grain balsa is used in boat decks for its excellent compressive strength, but if you have any water ingress i.e. through badly sealed fittings, watch out!! it sucks water and turns to a black mush very quickly. My opinion on its suitability for boards is based on the fact that the scrimmed sheets are composed of 1" square blocks, adding no longitudinal stiffness to the board and definately no ‘twang’, which I thought was part of the holy grail for us balsa sandwich builders. In the spoon kneeboards it may be used in the rails? instead of pour foam. Having used it on boats, I won’t be putting it in a board in the near future.



Sandwhich construction adds longitudinal stiffness, because the two glass layers are separated. If the sandwhich material has excellent compression strength it will keep the two glass layers separated nice and even despite heel slamming. End grain may not be as good as lengthways grain overall but it sure seems better than the foam sandwhiches.

Longitudinal balsa seems to have pretty good compressive strength, and ‘twang’. When I built some cedar strip cored yachts a few years ago, the longitudinal stiffness of the timber meant that we only needed to laminate with fibres in the 90 and +/- 45 degree axis’. surely this relates directly to Berts comments regarding timber over foam sandwich cores and flex response.

Is cedar commercially available as a core material or 1/8 inch thick boards?

Hey meecrafty.

When you are putting balsa onto a cured laminate a bonding agent is used, as I described in my last post. This bog is usually thick so when you are applying it to a vertical or near vertical surface eg topsides of a hull, it will not slide down the surface. The balsa does not absorb the bog very well. So if you roll a layer of resin onto the balsa prior to bedding it into the bog, The resin is absorbed into the end grains of the balsa. The two go off together and you get a strong chemical bond as well as a mechanical bond with the resin that is in the end grain.

End grain would not have been my first choice to build a board from. But as it was readily available and cheap, well why not. It is heavy, no twang that’s for sure. But it is fun to ride. platty.

you can buy cedar strips in 1/8 x 1" with bead and cove edges. I built a cedar strip canoe using these, but machined my own as the ready machined ones are too expensive. Western red cedar is availible in the uk from specialist retailers, should be more so in the US. It has very good strength to weight ratios and excellent adhesion to epoxy without too much resin absorbtion hence its use in canoes and yachts. The size we used on the cruising yachts was 2 x 1" with a tongue and groove edging, 4 people planked a 50’ yacht in a week or so !

Hope this helps