Ray K wrote:
They are discolored with vertical gray streaks on the sides, and gray
spots on the bottom where water flowing down the sides accumulate in
"puddles" along the bottom outside corner.

I've tried the following, applied with a saturated paper towel to small
test areas:
-- full-strength bleach
-- full-strength Simple Green
-- full-strength white vinegar
-- full-strength TSP substitute
-- 91% rubbing alcohol. Some improvement, but labor intensive to do all
158 feet.
-- paint thinner

I've also tried the following:
-- spray gas log cleaner
-- spray Fantastic
-- a mix of bleach with Oxydol laundry detergent, applied with brush.
Makes some improvement.
-- a mix of 1 part Simple Green, 1 part bleach, 2 parts water, applied
with a sponge. Makes some improvement.

I haven't tried ammonia, since I don't have any in the house.

I admit that I haven't had the patience to wait 5 to 10 minutes for any
of the above to work. I was hoping to find something that I could apply
with a dust-pan brush and have the streaks magically disappear
immediately. Any suggestions?

I have never cleaned any white ones but Lowes had a closeout sale for
some kind of plastic drip edge to snap on so they never get streaky
again. They had a big drum full of them but they are probably gone in
the trash by now.