I don't have the IPC document in front of me either, but I believe a good case
could be argued that the coating does not meet class 3 requirements due to the
necessarily diminished film thickness due to the bubbles, depending on the
amount present. I would like to add that bubbles are usually symptomatic of
improper reduction of the coating. Bubbles are generated whenever a coating is
sprayed on with traditional air-spray, air-assisted airless or HVLP spray.
They disappear unless: (1) the coating is too thick to let the bubbles rise to
the surface and pop prior to the coating hardening, (2) the reducer or thinner
used is evaporating too quickly to allow the bubbles to escape (even if it's
thin enough), or (3) heat is being applied prior to the solvent's escape from
the coating (insufficient flash time).
I agree that baking prior to coating is a good precaution, and with acrylic
coatings, it can be beneficial to coat boards while still warm. The trick is
to use a sufficiently slowly evaporating solvent.

Mike Moninger
Thermospray Company, Inc.
www.thermospray.com

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