small repair with the core

[shadowdancer]

repairing small areas (in the cockpit) that have old some rot , not much about 1 square feet. can I use exterior plywood coated with epoxy for core material

balsa only comes in 4x8 sheets and is very expensive

[GuyFromTheNorth]

When I did my lazarette lids I bought balso core that was cubed and backed with glass. It was dirt cheap and I could buy it in as small a sheet as I wanted. Plywood was 4x8 only. "Exterior" plywood can mean a few things. If we are talking pressure treated (green) plywood I'd be careful. It usually has a higher moisture content due to the process, which is fine for exterior use and rot resistance, however it will not let paint, let along epoxy, stick to it very well.

My balsa supplier:

http://compositescanada.com/

[psk125]

You can use whatever you want for core material, so long as it keeps the fiberglass laminations apart and keeps them from flexing. Problems come when using a different material from the surrounding area makes a "hard spot" because of the varying bend characteristics of two different adjoining core materials. To give an exaggerated example, if you used crumpled newspaper in one place and a thick aluminum sheet in another, after walking on the completed repair you would expect cracks to develop where the two came together. If you can keep the fiberglass from flexing by using thick enough laminates, like in cabin tops where plywood goes underneath winch bases, the different materials work out OK.

[Chotu]

You can use plywood and it doesn't need to be exterior grade, although that's not a bad idea. It just needs to be dry. It also doesn't need to be coated with epoxy prior to laminating. Your balsa surrounding it is not waterproof, so this little patch doesn't need to be either. What it needs to be is very dry, as a previous poster mentioned.

Also, you will want to do the laminating work when it is peak temperature for the day. Woods (including balsa) breathe with temperature changes. When it's getting hotter out, you'll get bubbles in your laminate, making for a weaker bond. When it's getting cooler out, you'll get some resin sucked into the wood, making for an even better bond than you'd have using foam core.

I wouldn't worry about dissimilar materials in this case. The stiffness of balsa on compression is pretty similar to the stiffness of plywood on compression assuming a similar thickness. Just make sure it's a nice, dry, stiff piece of ply.

[otterhound]

LBI in Groton, CT has balsa core in 2' x 4" sheets.
Rod

https://www.lbifiberglass.com/

[Down East]

I would not use exterior grade plywood.
I’ve re-cored decks and hull areas
It’s a messy tough job that you don’t want to do again - It’s not worth saving a few bucks.
I’ve used the glass backed balsa sheets (as well as other coring materials) - they’re easy to use and contour to match any deck bend. They’re also way easier to stick in place if you’re working from underneath (which you should do if at all possible).

[GRIT]

I agree with Chotu's post. I think it says all you need to know.

Cheers.
Paul.

[Chotu]

Originally Posted by Down East

I would not use exterior grade plywood.
I’ve re-cored decks and hull areas
It’s a messy tough job that you don’t want to do again - It’s not worth saving a few bucks.
I’ve used the glass backed balsa sheets (as well as other coring materials) - they’re easy to use and contour to match any deck bend. They’re also way easier to stick in place if you’re working from underneath (which you should do if at all possible).

I'd agree with all of this, but was just pointing out he could use plywood for the record. If that's what he's really set on doing. Actually much easier to use new balsa. But if he doesn't want to there is no harm in ply except as you said... If the deck has a camber to it.

[Scaramanga F25]

Cut correct thickness plywood into 2x2 inch blocks.

[bradfordharley]

If you only need a little balsa, go to your local hobby shop that sells RC airplanes. They are all made from balsa and they should have blocks or sheets you can laminate.