You never see the brown brown spots
Scrub clean one meh…
Although it looks like rust, the brown spots you see on the ship is actually not rust.
First of all, the brown spots are high up above the waterline.
The part of ships near the waterline is the part that gets repeatedly exposed to seawater, air and sunlight (which acts as a catalyst) and will rust first.
The brown spots on the ship we see from the video are actually just water stains from rainwater.
Rain water collects dirt particles in the air as it falls down, plus will gather any dirt on a ship's decks.
When it is still raining, the water flow is fast, so it doesn't leave too much stains.
However, when the rain subsides, the remaining pooled rainwater will often drain/drip slowly around the edge of window sills and other ledges.
This is when the dirt particles and minerals trapped in the rainwater have time to attach to the walls/hulls without being washed off by fast flowing water.
We see the same types of water stains on building walls, fibreglass ships and even cars.
So, these stains can in fact be scrubbed off as they are not rust.
But best done while stationarily docked at port by shipyard crews. You get untrained RSAF personel to rappel down the sides of a heaving ship at sea to scrub the hull, later have to do rescue mission. Or if they kenna sucked and diced into bits by the propellers, captain dunno need to write how many reports.