If this is a Taiwan
boat, the problems are probably low quality material. There is nothing you can do short of replacing all the metal with 316 stainless or bronze. In the interim, Oxalic acid will bleach out the rust stains.
Are these external chainplates that are bleeding down the hull
?? Whether these are external or internal, I'd be concerned with crevice corrosion
. Something that you can't really detect without removing the chain plate
. Crevice corrosions in the absence of oxygen but in the presence of an electrolyte (salt water) so are hidden under or behind the outside surface. Very insidious. Our Westsail 32 had very robust 1/4" thick 304 stainless external chainplates. They always looked pretty shiny and no sign of cracks. When we got back from SoPac, the insurance survey
reccomended removing and inspecting the chainplates for crevice corrosion
and our insurance
company required it. Thought they were nuts as the chainplates looked perfect. Long story short, removed the chainplates and found crevice corrosion after 4 years in tropical waters. The surveyor
said it was quite common. He'd found it so often that he reccomended an inspection
no matter what the age of the boat if it'd been in the tropics for more than a year.