If after 50yrs+ there is just a dusting of rust, forget about it...
- Black rust is millscale and actually inhibits further rusting
- Paints like hammerite can trap moisture if cracked (unlikely on an
RSJ)
- POR is good, but forget welding to it, or mag-drill n bolt to it
either
- Rust converters work, but only to a very limited depth
- Red Oxide does not have lead, probably some other oxide

Typically if you want to protect structural steel you...
- a) 3M Clean-n-Strip disc to a shiny surface including pits
- b) media blast to a "sa2" bright shiny surface including pits
- Paint with zinc epoxy primer £50 or epoxy mastic £30

To touch up coated steel, use galvafroid...
- Real cold zinc primer, £15 for 250ml, extremely heavy
- Only works where zinc can directly contact steel not rust
- No abrasion resistance, but very effective

I'd rather put the money towards replacing guttering early, because
that can really leave a trail of problems.