PDKL45

A little rust on the chain, cassette and other drivetrain parts is not of concern, really. It's light surface rust and if you're riding your bicycle, and wiping moisture off it after rides, especially in salted snow, it is fine. Keep your chain oiled--salted snow will make your drivetrain crusty within an hour or two, so you can carry oil with you on rides--and your chain will transfer a little oil to your cassette, as well as knock off rust with normal shifting.

Remember, rust, if dry, is usually fairly benign (rust inside your frame can be dangerous, because it's damp in there and damp is bad for untreated steel). Rust is just iron oxide, a chemical compound formed through exposure to oxygen in the atmosphere. A layer of oxide on the surface of steel--normal red rust--is actually helpful in some ways, in that it creates a barrier through which fresh oxygen cannot penetrate, protecting the non-oxidized steel below. Think of an old tractor or truck; often it will have chipped or scraped off paint patches. A thin patina of stabilized rust--a layer of oxide--will form in those places. The steel there is being dried often, so it is not of concern, just as it's not a problem to have a light layer of rust on your bike. It may look unsightly, but it's actually helpful in some ways.

Doug64

Actually, all of the inside surfaces of the bike's frame are accessible when you remove the bottom bracket, seat post, and fork.

General Geoff

The average cyclist will never remove the bottom bracket or fork from their bicycle. That's why I said "mostly"

TiHabanero

Was carless for 10 years, commuted by bike year-round, all four seasons here in West Michigan. Lots of lovely salt on the roads in the winter. Never ever treated the frame and never ever had any issues with rust in the tubes. For 10 years after that I had a car, but still commuted by bike year-round with the same nicely salted Michigan roads, except this time it was on a frame I brazed myself. Never ever treated it and I still have it sans the rear triangle. I bent those with the car. Looking inside the BB and down tube/seat tube there is only light surface rust just like the previous bike had. Pulled the stays off and cut them open, no rust.

There is absolutely no question in my mind that the narrative "rust from the inside" is simply that. A narrative. Perhaps to sell Frame Saver? If you are really concerned, mix automatic tranny fluid with chainsaw bar oil 50/50 and roll it around inside the tubes. Tranny fluid has anti corrosion qualities and the bar oil helps it stick to the tubing.