PocketShip.net

I watched a scarfing video (Oh God, has my life come to this?)- duckworksmagazine.com- and he put a thin coat of epoxy on the end grain of the scarf joint to seal it. It cured overnight and then he glued the actual scarf joint. His contention was that an unsealed joint might suck out the epoxy used for fusing the scarf and potentially weaken the joint. That seems reasonable.
Any comments?

Jay

That's pretty much standard procedure for gluing epoxy joints - especially on end grain. I usually don't wait overnight. Half an hour is usually plenty of time for the unthickened epoxy to saturate the wood fibers.

This question sounds like a pretty close analogue to the old "should I do a saturation coat of epoxy on my wood before laying fiberglass?" There are a lot of people who'll claim you have to to maximize strength and a lot of people who'll tell you that you're just wasting time and adding weight. Which is correct? In that case, a little of both. There are subset of fiberglassing jobs in small boatbuilding that require a saturation coat (like working with really, really thick 'glasss), but by a large it's not needed. The wood will pull in what it needs and so long as you've properly wet out the glass, everything'll be fine. The "saturation" coat is generally about uniformed boatbuilders being incorrectly conservative (epoxy is heavy and really weak in everything but shear...you need just enough to add it's magicaly ability to keep things together to the reinforcement material).

So, on to your question specifically. I think the proposed step is more effective at adding weight than strength. Epoxy is just a matrix material that is there to add shear strength between the layers (or in this cause particles) of reinforcement material. It doesn't take much epoxy to actually do this job. Basically, so long as all the little particles of thickener are coated in epoxy, you have enough. Epoxy that have been thickened to a thick honey or peanut butter consistency has plently of epoxy in it to do this, plus plently left over to get pulled into the end grain. So long as you start wil enough epoxy in your mixture, you're ok. To get into trouble, you'd basically have to start out wrong...like thicken your epoxy to the point that it'd wouldn't be spreadable (i.e. not useable) before you'd have to start worry about it not having enough epoxy for strength.

My $0.02, anyway.

But, if you decide to do it anyway, I'd go with wait 30 minutes suggestion. If you've gonna add weight that you don't need, at least don't waste time doing it.

The possible hazard of waiting over night is you can end up with a machanical bond or blushing. Dry fit scarf , mix epoxy resin, apply saturation coat. Then mix thickener with rest of resin and apply to joint, enough time. clamp and move on. Over clamping can leave a dry joint, just enough clamping preasure to hold everything in place.