Rust (2 Viewers)

[redolent]

I recently purchased a 2020 Land Cruiser that was very well maintained. However, it was driven through 3 northern Ohio winters and had some surface rust on the frame. I had a local auto renovation firm neutralize the rust with phosphoric acid and then covered and sealed with a paint like coating. The undercarriage now looks almost brand new. My questions are should I be concerned about rust inside of the frame? If so, would a product such as Fluid Film be worth the expense? My plan is to keep this vehicle for a long time and to not drive it during the winter.