Some bicycles get rode into or through lots of deep water, other bicycles often get left out in the rain and foggy weather.
Any which way, bicycles can most definitely rust from the inside out, so are there any recommended ways to protect against the elements?
Frame Saver is one example of an anti-rust product you can spread inside your frame: https://velo-orange.com/pages/how-to-use-frame-saver. Though as j4k3 mentioned these products are not perfect.
In reality, the rust in a steel frame isn’t usually bad enough to make the frame unusable. The bigger problem is interfaces between parts. Seatposts love to get stuck in steel frames that have been left in the rain too long. Screws, too.
Aluminum, titanium, and carbon frames will not corrode significantly in wet weather. In rainy areas you will sometimes find people who have a winter bike in one of those materials, and save their steel bike for the drier months.
Meh, I hear ya. My frame has had 43 years to show its ass and rust the bottom of the seatpost tube all the way through from the inside out 😞
Luckily my seatpost itself is made of stainless, so the tube itself never got truly stuck, but the original seatpost clamp bolt was a different story. That was totally stuck, and I had to cut that with a Dremel and replace the bolt and nut with spare parts.
Kinda tricky dealing with an antique BMX flatland bike that was literally rode quite hard and put away wet for over 20 years.