I'm talking early style visors here, most of which are sagging bags of
crumbled foam.

A while back, I wrote up a refurb experiment where I took some thin
craft foam and some cardboard and cut open the visor and built it some
new guts and glued it back together carefully with upholstery cement.

My brother, who lives in Austin and posts on the diesel list, came up
with a great idea: poke a hole in a visor and fill it full of
expanding home insulation foam, and use two boards to control the
flatness. I wish it had been my idea, it worked so well, but I will
share my technique. The results were excellent.

Remove the visor. Put a piece of clear packing tape down the "back"
side of the visor, the part you're not looking at when the visor is
installed. Cut three slits less than an inch long equally spaced down
the length of the visor. You will stick the nozzle of the foam can
through these holes. The idea is to fill all the compartments within,
and let the excess come out while a board is on top with a heavy
weight on top of that, like a box of books.

Use wax paper on both sides of the visor when it is on the table or bench.

The tape over the visor protects the surface of the visor from the
sticky foam when it expands and comes out the slits. Pick away the
foam in a hour or so, then peel off the tape. You can scrape off what
little residual foam there is and or wash/scrub away with mineral
spirits before the foam sets up too hard. the slits you cut will seal
themselves, you'll not be able to find them when you are done.

The result is a uniform, strong visor more like the later ones. The
whole project is less than ten minutes of work and four dollars in
expense. One can will easily do two visors. If you don't get all the
empty pockets the first time, cut another hole and try again.

Jim