Mild steel on deck

[GHA]

Having built a steel 40 footer I can tell you if done properly it will never rust unless damaged. However, stainless will still be best. 304 is fine above the water-line as I know from 40 years of using it!

Yes, "if done properly". Getting rid of mill scale by hand ain't easy.

And "never rust" is a bit optimistic, sooner or later the paint system will start to break down and you'll be thinking about a repaint. From living on a many decades old professionally built steel boat.