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I read John Barrett's post on surface preparation for bonding noting his observation that for dental cement, a fine abrasive making more surface area is now preferred, providing better bonding.  This supports what I heard at a composite repair course put on by a company that repairs composites for airliners.  They also recommend using a fine grade sand paper (240 to 320 as I recall) to provide maximum surface area with minimum damage to underlying fiber.  They wipe with solvent (twice_ prior to sanding and vacuum with a clean brush to remove dust.  Bonding within 4 hours of sanding is recommended because the just-sanded surface bonds better than one that has been sitting for a while after sanding.  (It was described as a "high energy" surface.  I leave explanations to the surface scientists.)  Grinding away with 40 grit seems intuitively superior, but apparently science, experience and testing have proven otherwise.

Fred

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