On Sun, 13 Jul 2008 17:11:42 -0700 (PDT), "
wrote:

Sanding beaks the fibers producing a less strong lap joint.
So what if you use good one side plywood - is the lap joint going to
be weaker because it is sanded?

I think that this old bugaboo might have been valid back in the days
of resorcinol formaldehyde adhesives but I havent found it to be valid
at all with epoxy.

sanding breaks the fibres.
well what do you think cabinet scrapers do???
that is how you end up with a smooth surface. you abrade/cut/grind
down anything poking above the surface.

plywood lap joints are usually finish sanded and it is necessary to
have the surface of the lap actually flat on both parts or you will
get a weaker joint.

in any case none of this sanding vs cabinet scrapers vs broken glass
argument matters a toss.
make some standard test pieces by whatever method *you* use and after
letting the adhesive cure off, break them.
if they break through the joint the glue work is useless. if they
break through the wood and never through the joint then the glue work
is ok. end of story.

I sand every joint face in every piece I glue and never ever have I
had a glue joint break in a test piece.

the important things in joining with epoxy arent sanding or scraping.
they are....
absolutely no oil contamination of the joint.
no wax contamination.
no water contamination.
both pieces of wood at the same moisture content.
no loose dust in the joint area.
epoxy correctly mixed.
epoxy joint clamped in position with an appropriate gentle pressure.
joint not clamped so hard that the glue is squeezed totally out of the
joint.
absolutely no joint movement until the epoxy is fully cured.

and lastly but most importantly
cure the epoxy in the proper temperature range.

note that I sand with 80 grade orbital sandpaper glued to a flat block
and my final sanding preparation is done by hand. I achieve a pretty
nice surface.
if you machine sand and burn the surface dont expect a serviceable
joint.

ymmv but do the test pieces!
Stealth Pilot