yeh, i've heard they go over 100,000 miles without breaking.....
Boiled linseed oil does work, can also use on nipples as threadlocker (kinda messy). Can buy this in any hardware, walmart etc in paint section. It takes a long time for b.l.o. to dry hard (at least a week) and it has a unique, not-too-pleasant odor while its drying. So, treat frame, then hang up in a shed for a few weeks before you try to build (this is very messy proposition on already-built frames, wipe off spills as this stuff is a gummy, tenacious mess to remove when dry). A quart/litre can will do about 10 bikes.
Steel frames often rust on the inside. The inside of the tubes are raw steel exposed to ambient conditions, and unless they've been treated with a rust inhibiting coating, then some rust will eventually occur. Water gets trapped inside frames and can't readily exit/evaporate. A normal point of entry is rain hitting seatpost runs down inside seat tube, pools above BB shell. You'll normally see some rust at the bottom when you look down a seat tube. Always avoid immersing your hubs, chainstays and bottom bracket in water (stream crossings, etc) - once water gets in it usually causes some problems - rust in hub/bb bearings/races, rust in chainstays.
Oh, I just reread the OP post - if you're shelling out for a rivendell just go ahead and pay them to rustproof it for ya - tell them before its painting time!