rails tend to rust where the fabric/plastic hold moisture up against the frame. Use stainless cleaner and protector to keep the frame clear of rust eg this stuff
. The vinyl window looks pretty discolored, if rust satin remover (oxalic acid) does not work
, you may have to remove that panel and stitch in a new one. I think that bimini frames are made from cheaper or lower grade stainless and so surface rust stains are more prevalent.