On Mon, 26 May 2008 05:35:07 -0700 (PDT), waynard
wrote:

Hi,
this is a bit of an unusual problem. I recently bought a boat and it
came with a sonar unit that has a weird kind of mottling or splotchy
film of some kind on the screen. Almost looks like dried seawater
stains. It looks like you could almost scratch it off with something,
but of course, you risk damaging the plastic screen.
Any ideas what it is or how to remove it safely?

Try wiping it with a soft cloth dampened with vinegar.

Casady