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Dana  Westphal refers to the manual calling for a reverse order to the prep
process from that expounded in the LML; that is sand first, then use MC.  He
also talks about 80 grit and 40 grit sandpaper being called for in the
manual at different times.

I haven't seen the call for 80 grit - only 40 grit in my IVP fastbuild
manual.  Not to say it isn't there, I just haven't seen it.  I do know that
in dentistry (if you'll forgive the analogy) we used to use very coarse
abrasives for retention.  Example: use a coarse diamond stone to rough up
the surface of a tooth to help retain the crown with the cement.

As technology and understanding has progressed,  dentistry has discovered
that those efforts were counterproductive.  The amount of retention is
directly proportional to surface area.  Surface area is greatest with the
finest grit of abrasive material.  We now use micro-etching which is
sandblasting with 5 micron aluminum oxide to rough up surfaces prior to
bonding.  Intuitively we want to see some rough gouges in a surface to
convince ourselves that the bond will be strong, but actually a very fine
frosty appearance will provide more of that surface area for the greatest
bond.

As for the sequence of events, we micro abrade last - whenever possible -
except where we want a chemical bond to be enhanced by a surface activator
we can paint on to the bonding surface.  These concepts generally apply to
bonding of metals, porcelains and composite materials that we use.

I am not a scientist and have no knowledge that these principals carry
accross to the world of fiberglass and resin bonding.  I only know that this
is the current understanding in dentistry.

Regards,
John Barrett


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