I'm not familiar with fish oil. Is it like naval jelly?
The rust converters seem to work best if you also seal them with something like POR-15 or Zero Rust. My old daily driver was painted in the autobody class in the vocational school in our school system. They used a converter and the rust came back around the original spot in 2yrs.
The rattle can rust converter is more for surface rust applications, that way you don't have to sand the rust off before applying primer paint. If the rust is more than surface rust try nocking it off with a steel brissel brush, if it looks like a scab the converter wont really work very well. Hope this helps. 0/