Here my Rat Rod Cruiser, "boardtracker" frame, rust conserved wit linseed oil, a fork from a 50s motorbike, wheels and fenders from an old swedish military Kronan.
I coated the rusty surface only with cooked linseed oil. After a while the oil hardens completely and protects the surface. This method is also used to keep the patina of oldtimers, old machines, fences, weapons... in 18th and 19th century it was the basis of any paint. I think on rust it works and looks better than normal clear coat. The bike was mostly outside for one year now, in wet condition, and I could not see any changes on the surface. But the frame is made from quite thick consruction steel, so I don't worry much about it anyway.