The stainless steel Tubus Cosmo rear rack looks quite pretty on my old utility bike, which has been used in all weathers, including in coastal areas with a bit of salt. Just a slight bit of discoloration on one of the welds. The alloy shell on the S5/2 hub gear I fitted to this bike is discoloured from salt corrosion after 44,000 kms of use, but it still functions OK.
An ordinary steel rack such as Thorn's own brand gets rust spots caused by the pannier bag atachments rubbing the paint off. It may not look pretty when the bike is ridden without luggage, but I don't find it a big deal, the small amount of surface rust doesn't impede function and the rack just gets the same treatment as the frame and forks. On these, from time to time I rub down any rust spots I see, and touch-up the paint. At much longer intervals I strip the bike down completely and repaint the frame and forks (and any other painted parts) completely.
My old utility bike frame dates from the mid 1960's at the latest, it could be much earlier, maybe even from the early 1950's. And I still have my 1977 Woodrup steel frame derailleur tourer, which has had the same "touch-up paint, very occasional complete repainting" maintenance regime for the external frame/forks, plus an internal treatment with a sticky rust-inhibitor a few years after purchase.
So rust isn't a major problem for me, it might be different in areas where salt is used for long periods in winter, if you ride in coastal areas in bad weather (salt spray), and for bikes stored outside for long periods.
If you build up the bike yourself, it is easy to treat the insides of all accessible tubes with some kind of rust preventer after purchase but before doing the build. But I think Thorn frames sold nowadays get a fairly good rust prevention coating anyway.
Facing and cleaning out the threads are operations you do on a new or repainted frame before building up the bike. Thorn offer them as an optional service called "workshop preparation" when you buy a new frame from them. I took this option for the 3 Thorn frames I bought unbuilt, as I reckoned it wasn't worth buying the tools to do the job myself. They do this automatically when you buy a complete bike.