We occasionally get questions about internal rustproofing on steel frames. Should you worry about internal rust on your new, or used, frame? The fact is that the vast majority of steel frames never get any internal rust proofing treatment, yet last for many many decades. On the other hand, you'll occasionally see a rusted out frame, particularly in regions where road salt is used. So to be on the safe side, I rust-proof all my new frames. You'll certainly want to rust-proof any frame as nice as a VO frame.
Back in the day I simply sprayed WD40 into the frame, but today I prefer Frame Saver. Frame Saver is designed to get into the smallest spaces in your bike frame and leave a protective coating when it dries.
A single can will do several frames, and application takes only a few minutes. The included directions are pretty good, but here are a few photos of Scott rust-proofing a prototype: