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Replacing balsa core

2K views 15 replies 9 participants last post by  colemj 
#1 ·
This has been gone over and over again. Just wondering, one thing. Once the old core is stripped out, is the new balsa core to be wetted into place, as in fully saturated in the resin?

Or stuck into place with resin, then covered over with resin, and wetted fiberglass?
 
#2 ·
you should flex out the balsa and coat in resin, then put in place. The idea is to seal the balsa on all edges before you place it. This is a future proofing thing... so that if water does get ingress, its harder for it to migrate, as each 1x1" block is basically sealed into itself.
 
#7 ·
Given that the balsa is laid end-grain, I wouldn't have thought there would be much migration cross-grain. Does the core not come from the maker in a sheet with the blocks already glued to each other?

Wow, showing my age here - I haven't worked with balsa core since the mid-seventies when we used it putting roofs on reefer truck bodies.
 
#10 ·
No - they come lightly glued to a scrim just to hold them into a sheet. As noted earlier you should fold back each seam and brush on epoxy to seal it all up - the old "no cross grain migration" story is BS. Unsealed balsa will soak across the grain. Just ask anyone who has ever had to do a re-core job.

It's best to roll epoxy onto the faces of the sheets of balsa as well - it doesn't soak up a lot but if you don't it can absorb enough from the wetted fabric to starve it a little and weaken things.
 
#9 ·
Hooker,

Pick up one of the Don Casey sagas. He is like the Yoda of boat repairs. Hard is the rock that moves not, but easy the repairs will be. Do or do not. There is no try.

You can find his wit and wisdom all over the web. Plenty of books too. I have a few but not the full Canon.
 
#11 ·
Ok, so I used laminating resin... I hear epoxy being thrown around. I know my resin was 2 part, but that was the only similarity to epoxy that I know...

Did I use the wrong stuff? Dried hard as a rock, so I thought it worked well.
 
#12 ·
'Laminating resin' is generally unwaxed, air-inhibited orthophthalic polyester. Which is what your boat was made of originally, & it is fine for this and many other jobs on a sailboat. Uses a small dose of catalyst (MEK peroxide) and tends to kick pretty fast. Its styrene solvent is noxious and potentially explosive.

Epoxy is a different catalyzed polymer, bisphenol-A resin with amine crosslinkers. Little to no petrosolvent added, less stinky, non explosive. May cause acute dermatitis in sensitized persons. Also works just fine for boat repairs: as a rule, it has better secondary bonding qualities (sticks to cured or mixed surfaces better) than polyester resins, is less brittle, and in its medium to slow forms may give you more open time for repairs. All of which make it worth the higher price tag for many of us doing boat work in a non-production setting.:)

One mildly important distinction to make: some chopped strand mats rely on the styrene in polyester resin to dissolve the glue holding the mat together & won't wet out transparent in epoxy, tho I've read they still perform okay structurally together. I guess if you choose epoxy and want structural performance, best to use something other than mat anyway. Epoxy-compatible mats are out there, and biaxial fabrics that include mat rely on stitching rather than styrene glue, so they are good with epoxy.

Pros would likely do a recore with polyester. They buy it by the barrel, they work fast, and cost differentials matter to them. For my boat work, epoxy is a better choice. But either will do the job.
 
#13 ·
Good post Bob - I would only make one small change - most mat uses styrene soluble binders. If you are using epoxy you must specify to your supplier that you want epoxy compatible mat.
 
#14 ·
OK, now to open the can of worms...
I've heard gelcoat does not stick well to epoxy, is that true? Only reason why I used laminating resin.

Setup times on the laminating resin was quick like minutes, not hours. All this stuff was toxic as heck.
 
#15 ·
Gelcoat = polyester resin + pigment. Like any polyester resin, it sticks so-so to anything other than uncured polyester resin. Sticks beautifully to that. Epoxy is more flexible than gelcoat, which can lead to cracking. Add the possibility of amine blush on the epoxy, and you might have trouble getting that perfect feather-in or a durable repair, putting gelcoat over epoxy. People have done it successfully, but I'm not sure what tricks are required.

We did our recore from below (tons-o-fun), so gelcoat & cosmetic blending were not an issue. I hear gelcoat smells like a hippo's armpit.
 
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