> Two weeks ago, I discovered what appeared to be about a 1” “crystallization”
> just visible by looking through the Plexiglas at an angle within the carbon fiber
> “sandwich” holding the door window in my Lancair IV-P.

Robert: sorry to hear that you will have to cut up your door. That's a
lot of work,
but better than suddenly opening the window in the flight levels.

Two things come to mind as possible triggers for a crack in Plexiglas.
The first is a scratch or dent at the edge of the Plexiglas that can act
as a stress concentration. Before installation, the edge of the window
should be as smooth as reasonably possible. Any saw marks should
be sanded out. Any small cracks at the edge should be completely
removed by sanding away that area.

The second is solvent exposure. Do not use acetone, ethyl acetate, toluene,
or lacquer thinner at any time in the immediate vicinity of the Plexiglas.
Weeks, months or even years later, crazing can develop in the areas that
were exposed. I haven't looked up all the possible materials that are
incompatible
with Plexiglas, but I would also suspect MEK. Aircraft paints are likely to
contain these or similar solvents, so be sure to keep the paint solvents
apart from the window. Mask carefully, and remove the masking as soon
as possible after the paint tacks up.


-bob