Most boats get small rust dots on their decks due to water dripping off of rusty hooks or tools, etc. However, boats near railroad tracks or metal construction, even work on your own lift or on your boat, can become covered with numerous small particles of metal. Then when it rains, the metal becomes rusty and leaves a rust stain surrounding it.
The boat I will highlight below is docked near a railroad bridge that is being improved for high-speed trains. The work threw clouds of metal particles into the air that can coved the boat. Here in Florida all boats near railroad tracks are susceptible to a dusting of metal particles. The boat I show below was completely covered in rust “dots;” it was a 44 foot catamaran motor trawler. It took 20 hours to remove all the rust and metal particles from the gelcoat and stainless.
The process to remove the rust consists of two steps. First we apply a rust remover to remove the rust stains. Then we remove the actual metal particles. The process takes all the wax off the surfaces and dulls the surfaces so that the surfaces must be compounded to restore the shine.
The yellow/orange in the pictures below is due to millions of dots of rust stains from the metal pieces. The close ups show the individual dots.
Up close 
Metal pieces fall on stainless as well 
Another close up 
