Dear vjp2...:

On Mar 24, 11:03*pm, wrote:
Sometimes opaque white plastic bags form a black
smokelike smudge.

What is this? Can this develop on coroplast.com's
plastic sheets?

Plastic films are prone to developing a static charge. This will
allow them to attract and hold everything from mold spores to soot /
smog.

I am trying desperately to determine that this is not
mold.

I recommend you find out what it is, rather than forcing it to be
something you fear. Swab it and look at it under a microscope. Any
regularity to the "grains", any appearance of biology?

It makes no logical sence that this is mold or that
mold could form on this corner of the coroplast
and not in other parts of it.

It might simply have been exposed to the environment in a different
way. Life rarely "makes sense". Look and see. My penny's worth of
free advice.

David A. Smith