My understanding is that those types of "high fish oil" paints are alkyd paints that contain a lot of oil which is supposed to penetrate the rust and stop the oxidation. I think they work pretty well on surface rust but I don't think the would work on rust scale, there would simply not be enough oil to saturate the rust scale.

As with any paint job the key is surface preparation. If it really thick you might want to try Naval Jelly, which is a type of Phosphoric acid that "eats" the rust so it can be scrubbed off. It may require multiple applications. The black ferric phosphate coating left provides a degree of corrosion protection and can be painted, in fact it enhances the bond between the metal and the paint similar to the industrial process called phospherizing.