Try JOMAX .. available at ACE & both Borgs .. .. 1 part JOMAX + 4 parts
bleach + 20 parts water ..

Spray it on .. watch the stains go from black to gray to orange to
yellow to GONE.

Rinsing is optional, but I usually do it just to keep any nasty chemical
reactions to a minimum.

This stuf is GREAT and has to be the best kept secret around.

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.

em.