: Hmmm... OK, I'll buy that. I have heard of someone snapping an untaped butt
: joint when moving it around during building. But that was most likely
: before the epoxy fully cured. I also like your theory on the lighter
: coloring. Pygmy's explaination is oxidation, but, especially in your case,
: greater absorbtion seems more likely.

Keith

When building my Rob-Roy canoe I decided to alter their plans for a plywood butt-block and use glass patches.
Due to my inexperience with F.G. my first attempt ended in snapped joints. I cut some 8" squares and glassed both sides of the joint, let each side cure for a day before moving. It worked fine until I started putting the boat together. 2 of my 4 joints done that way gave out.
Reason: I had cut the squares out of 3.25 oz. satin weave cloth and laid them over the joint with the weave running with the joint. It was great until I actually picked up the long panel and it wouldn't hold the weight. There were only the fibers running across the joint to hold it as the others were running parallel to the joint.

I didn't change my plans, but sanded this glass cloth down and redid the joints using two layers of that same 3.25oz cloth only cut and applied in a biased direction. I feathered both layers slighly and when the boat was assembled and the hull glassed, you can't see the glass nor any raised areas in the finish. My joints have 9.75 oz. of glass inside and out and are very strong. There's no indication of any flat spots in the panels from this method either.

There is NO discoloration in any of the panels since I pre-coated all the plywood panels before assembly and when feathering the glass, didn't go through that saturation coat.

Just another method that seems to have worked very well.

The picture is the finished joint with 9+ oz. of glass (two patches of biased cloth across the joint) inside and out.

Rehd