My first suggestion would be that you only glue up two boards at a time.
Example in your case, glue up two of the boards that are going to make up
one side of the panel. Glue up the other two side boards and the two middle
boards also if you have the clamps for it. After at least a couple of hours
glue two of the sections together then after a couple of hours glue up the
lining up and keeping six boards slick with glue lined up as you apply clamp
pressure can be quite a chore.
I'd have to say that no it is not usual for the jointed boards not to fit
perfectly. I consider two boards ready to glue up when I stand them on edge,
one over the other, and can't see any light shining through the jointed
edges. Neither biscuits nor clamp pressure should be used to "cover up"
badly jointed boards since it introduces stresses to the glue joints.
There is a caveat here though. Though I don't practice it myself there is a
school of thought that holds that one has a very small gap at the middle of
the two boards and when they are pulled together by clamps it puts tension
on the ends of the joints which is a common spot for panel glue ups to fail.
While I don't practice that method I see nothing wrong with the theory.
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