I don't know how the glass is but with sand and moisture I'm with Dough.
If there is some sand collected somewhere it collects moisture and rusts
I did a lot of rust repairs for a couple of decades and found a lot of moisture traps.
I did a 60 impala 2 dr ht that must have been some Beach Cruiser in California.
Floor inside the car and trunk was like new with nice white original paint
Roof above driprail was like lace due to sand accumulated under the chrome trim
and on the inside of the roof Wheel well openings was filled with same white fine sand
and rusted out
Normal for a 59-60 chevy is they rust in bottom of A-pillar in what i call the compost
where leafs and such from air intake gathers, The floor , trunk floor and rocker panels.
This one was like new there
And nothing beats the feeling riding in a sandblasted convertible..the crushing sound of
sand between your teethand the blinding pain of sand in your eye when you hit 50 mph