Balsa Core Sealant

[mmullins]

I cut a 4" hole in my swim platform to install an observation deck plate. What is the best way to seal the exposed balsa core before installing the plate?

 

[Flatswing]

Rod Collins (Marinehowto.com) has a nice explanation of this. Basically you need to undercut the FRP to remove at least 1/4” of all the peripheral core all the away around. A key hole bit in a Dremel type tool is an easy way but with a 4” hole even a small sharp chisel may work. Most importantly meticulous prep of both under cut FRP skins before sealing with thickened epoxy (duct tape on the under surface of the swim platform will help contain the epoxy peanut butter). Once hardened, you can clean up any high spots with the hole saw but do not accept any voids.

 

[sean9c]

A hole that big is easy. Cut back the core a 1/4" or so, with that big a hole you can just reach in with a razor knife and cut away at it. Then fill the space you've made with thickened epoxy.

 

[Comodave]

I have the bits for my Dremel that Rod recommends. Or you can use an allen wrench. Cut the short side down to about 3/3” and chuck it into a drill. Then use the short end to reef out the core. Not as neat as the Dremel but it will work. Then paint the exposed core with regular epoxy. Let it soak in for a few minutes and then fill the area that you reefed out with thickened epoxy.

 

[DDW]

One important point: When you mix the thickened epoxy, do not 'whip' it, which will entrain air bubbles. This can cause enough porosity to allow a water path through the epoxy. The thicker you get the epoxy, the easier it is to work with, but also the more likely you have entrained bubbles. A good practice is to paint some relatively fluid epoxy onto the core (not likely to have bubbles) then mix more thickener in and putty it up. Ideally you vacuum degass the mix before applying it, but most people don't have a vacuum degasser in their garage.

Learning this fact cost me many thousands of dollars.

 

[Figment]

I've owned enough boats which were previously owned by people who didn't bother asking this question to say: GOOD ON YOU FOR ASKING THIS QUESTION AND DOING IT RIGHT.