Do anti-corrosion primers actually work below waterline?

[Ship'sCat]

I have a few corrosion spots appearing through the paint on a cast iron keel and a steel plate fin on my boat. The corrosion is in awkward places to remove, such as recesses for bolt heads. Does anyone have any successful experience in using any particular product promoted for painting over rust as a primer below the waterline in salt water conditions? Or is it a matter of just having to take the rust off anyway for a proper job?

 

[alahol2]

I've been using Fertan for years. I still get rust spots but I don't know if they are in the same place as previosly so I just keep treating the spots. It never gets out of hand so I'm content.

 

[jwilson]

Scrape/sand chip/wirebrush as appropriate then Fertan then zinc-rich primer then whatever paint you want to apply. Not perfect, but as Alahol2 says keeps it under control. I've never tried a "paint onto rust" product on the boat but the one I lazily used on a wrought iron garden gate was spectacularly useless.