Epoxy Question

Say you put some epoxy and fiberglass in your hull in the evening and the next morning discovered you forgot to glue something in (lets assume 10 hours have passed at 70 degrees). The epoxy isn't sticky to the touch but hasn't fully cured yet. My question is: If you mixed up some thickened epoxy and glued the missing item on would you still be able to get a good chemical bond or would you need to rough up the spot so you get a good mechanical bond?

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RE: Epoxy Question

Resin and hardner used might matter. I wouldnt count on a chemical bond if its not still rubbery.

RE: Epoxy Question


First, try a little sanding on a small spot. If the epoxy's fully cured, you'll get dust. If not, you'll get little gummy balls. if it's gummy you can glue with no further sanding. If it's dusty, keep sanding and rough it up before gluing.

Second, it's all a chemical bond. There's no such thing as a mechanical bond for epoxy. The difference is whether the bond is uncured epoxy linking with itself to form (or continue) a polymer, or uncured epoxy linking with the active parts of a foreign molecule. The more active the foreign molecule (up to a point) the better the bond.

For materials that are not as active, the bond strength can be increased by increasing the bonding area. That's what roughing it up does. It's the same as how an 8:1 scarf is stronger than a butt glue joint.

The other thing that sanding does is expose more active areas. Since a material's surface is exposed to oxygen, it may have reacted and not be active anymore. Cleaning off the outside surface exposes fresh material that is ready for bonding. It also removes pollutants, like water, oils, etc.

Finally, the difference between the 2 bonds is not really important in the boats we build here. They don't get the kind of stress that needs every last bit of strength that can be squeezed out of a bond. As long as the bond is stronger than the wood, the wood will break first anyway.

Apologies to all the real organic chemists for the over-simplifications,


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