Frame Saver is an aerosol that coats the inside of a steel bicycle frame to prevent rust. Carbon fiber and aluminum frames comprise most of the bicycle market share these days — have for a while — but many cyclists still prefer the ride quality of steel frames. Also, the fixed-gear trend has granted a second life to thousands of ’80s-era steel bikes over the past few years.
While the paint on the exterior of the bicycle frame’s tubes prevents rust, there’s no rust-protection on the inside unless Frame Saver, or something similar (such as Boeshield T-9), has been applied. It’s arguable that if you take good care of your bike, pull the seatpost and let the bicycle dry upside down after rain rides, rust won’t be a problem, but using Frame Saver is inexpensive insurance for what can be a costly investment. I’ve applied it and found it solidly in place, with no trace of rust, many years later.
The easiest route is to apply Frame Saver (or have your local shop do it) before your bike is initially built up. A can generally coats two frames, and it can extend the life of your frame/fork indefinitely. If you’re going to use it on a bike that’s already built, it’s best to strip the bike completely before applying; the bottom bracket and headset regions are particularly vulnerable to moisture.
Peter Weigle, the manufacturer, is a highly regarded framebuilder and has posted some beautiful photos of his handmade bikes on Flickr.