This is what I suspect as well. Surface rust on SS shows up like freckles or spots all over the surface.
The OP is reporting staining on the fiberglass around the base of the stanchion.
My experience has been the stanchion bedding has failed. Water is wicking underneath the stanchion. The water and lack of oxygen is causing the SS bolts to begin to rust. eventually, water wicks in and out and creates a rust water mixture, which then evaporates and leaves a small rust stain around the stanchion that looks like a coffee ring around a cup.
The resolution is to unbolt the stanchion, clean the area, replace the bolts, and then re-bed the stanchion.
No the rust is formed by the little pieces in the rainwater. They drip down to the bottom of the stanchion where the rust stays when the water evaporates.This is what I suspect as well. Surface rust on SS shows up like freckles or spots all over the surface.
The OP is reporting staining on the fiberglass around the base of the stanchion.
My experience has been the stanchion bedding has failed. Water is wicking underneath the stanchion. The water and lack of oxygen is causing the SS bolts to begin to rust. eventually, water wicks in and out and creates a rust water mixture, which then evaporates and leaves a small rust stain around the stanchion that looks like a coffee ring around a cup.
The resolution is to unbolt the stanchion, clean the area, replace the bolts, and then re-bed the stanchion.