flynnibus
Premium Member
Rust on both.
More like water stains.. look, the stains are all below the cutout holes in the beams. If it were surface rust, the rust would not just be 'running down the side' when the water doesn't stay there anymore than anywhere else on the surface.
Those are stains from the drains. Any rusting would be up above on the deck level with the stains coming from minerals in the drain water accumulating there on the sides.
Given the discolorization, this looks more like muck stains to me.