Mac,
They are stainless steel walkways, and if you're applying them to covered hoppers, leaving them unpainted will be appropriate for most prototypes.
If your prototypes do require paint, you might sand them with 400 grit paper to give the surface some tooth. I paint with solvent paints, such as Testor's Model Master line, Scalecoat II, Floquil, so paint adherence isn't much of a problem at all, though you may have different results using acrylic paint.
Unless you're painting them yellow or other light color, I wouldn't worry about primer. And I'd only use it as a color undercoat to make the color coverage more complete, but not for any other reason you might have for using primer.