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As the thread title suggests,

What are your thoughts on core bond putty...retail premix stuff like ATC Core-Bond, or just mixing your own putty?
ATC Core-Bond up here in Canada is pricey....$210 for a 5 gallon pail....but I think spending the extra cash for it would be worth it due to it always being consistent, and in my Viper rebuild, I have rough estimated that I'll be bonding approximately 50 square feet of balsa core, and just over 4.6 sq feet for the transom.

Trying to remember what ratios of cabosil (fumed silica) and fibers mixed in with resin to the consistency you like in your first batch might be problematic to duplicate in your subsequent batches.

If mixing your own putty, what ratios of cabosil and milled fibers works best?
Does resin type affect this? ie regular polyester versus vinylester?

I didnt use core bond myself but when it was redone the second time, Brian did use core bond. For my transom though we use resin and Cabosil and just thickened it up until it wasnt runny what so ever and used a trowel to spread it on between the 3 layers of 1/2 plywood (yes, its marine grade ply) and the then the outter skin too. We didnt measure the amount of cabosil each time, just thickened until we were happy. I can see where the core bond would be better suited for doing the core like you said though. As for the resin, I just used the plain old polyester resin. My whole Vulture took about 15 gallons in all (3, five gallon buckets). That was everything.

Random question - was there a reason why you didn't use fibers in your putty for the transom portion? Simply not needed due the layers of glass on the inner side?

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If you are referring to the milled fibers, that stuff essentially does the same thing as Cabosil. As for the putting fiberglass mat in between the layers of the transom, 2 layers of CSM were put on the outter skin and then troweled on some resin with Cabosil then put the transom in and I put a layer of 1708 in between each piece if I remember right. I didnt say in the post but you can see it here - http://www.hydrostreamforums.com/viewtopic.php?f=13&t=1171&start=40#p9924

Hope that helps. Its been a while since that part was done so my mind is a bit fuzzy on what we all did. lol I do know its strong as crap though.

I prefer core bond over all other materials for bedding coring and plywood.

Before I get started, I would like to add that it is important to prime your plywood to seal the fibers. If you don't, it will pull resin from your layup and or not bond properly to the core bond. I like to add 10% Styrene (or acetone if needed) and double the MEKP hardener in the resin for priming.
If using un-primed balsa, I seal both sides the same way as the plywood. The balsa fibers are like bamboo. They have dividers in the strands and the open ends will wick up resin and moisture. With the wood products primed this way, the resin absorbs into the fibers and creates a superb bond. (We probably have seen fiberglass peeling off wood because it hadn't bonded) You are now tying into the wood at a cellular level.

We all know that a resin rich lay-up is brittle and a resin starved lay-up is weak and flexible. With that said, after grinding the surface, you will have slight highs and lows correct?

OK, here's my take on wetting out some csm (chopped strand mat). Just like I said, the highs and lows under the coring or plywood will create rich and starved areas. OK, under compression this may not be a big deal. You definitely wouldn't want this condition on the side of a stringer. I just don't like this and it's heavy.

The DIY putty. OK, it's dense and won't compress. It's brittle but there shouldn't be any flexing,.. so far so good. It's heavy and there isn't much in the way of "adhesive" qualities to resin putty.

Core Bond.

Ahhhhh Good ole Core Bond.... It is light, an adhesive, a fairing putty, Same strength thick or thin, easy to apply.

Here you can see my primed plywood getting bonded to the transom. Look closely at the blue core bond, see the solid circles? That's a slight depression from over grinding. The putty has filled this void and I haven't lost any strength here..



Core bond used as a fairing compound between the floor plywood and balsa floor panel over sponson.



Core bond in a zip lock bag for setting "primed" plywood floor.



Core bond as an adhesive to further bond the gaps in the floor to the core.



Then a spreader flattens it out before glassing in.




I'm not saying the other ways will fail. I'm a core bond guy and these are the reasons I like it.

My mentor builds boat molds and boats from his molds ranging from 8' up to 32'. He understands fiberglass materials at the molecular level. I am in awe every time I go up to his shop. Maybe in another thread we can discuss the various resins and fabrics and when and how to properly use them. What is proper "tooth" after grinding for best bond? I will be glad to share what I have learned..

Thank You for posting.
After you put the bond down then the primed balsa do you use sand bags or do you have a different method ?

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Also what type of resin do you prime with? I was thinking of priming the bare hull with epoxy before laying in balsa( To create a moisture barrier). Is this correct and will the core bond adhere to epoxy?

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Thank you for giving me your time and knowledge I have very limited experience with fiberglass work and your posts are motivating me . It clear that you do quality work.

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Hi Mike.

You can epoxy over fiberglass (Polyester/ Vinylester resins) but not the other way. They won't adhere to epoxy's. The properly ground (16-24 grit) surface will be perfect to accept the core bond. You can fill your lifting strakes with the core bond then put core directly over that.

The core bond has got enough adhesion to hold the coring without weighing down.. If you feel some movement, by all means, add some weight.

I try, at all costs, to avoid bulk packaged resins such as BONDO brand. If you have a choice and can get to a fiberglass supply store, Ortho polyester laminating resin is just fine.

There is ISO resin and Vinylester also but that's for other applications. Yes, they could be used but more expensive.

I've been reading so much stuff and some of it is conflicting.
So I should
1- grind the surface down with 14 to 24 grit.
Then vacuum and clean with acetone ?
Then apply the core bond and put the resin coated core down on this?
Or do I also have to coat the original hull with resin before. Also when I lay in the core is the resin on it still wet or do I let it cure.
Sorry for my ignorance on the subject. I do appreciate your patience.

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