How can I tell the difference between rust and pitting?

I assume there is no way whatever to remove pitting?

Also, I am wondering if the product metal rescue actually works? I would like to try it but am rather hesitant because of the $25 per gallon cost.

Rust produces at least microscopic level pitting. Pitting can be from rust, other corrosion or mechanical action. Metal Rescue and similar products only change the rust to something else which will not further corrode. Some products include a protective coating as part of the process. NONE of them will restore the metal, so any pitting present will remain.Pitting can be removed by grinding, but that is not always appropriate depending on the situation. It would help us greatly if you disclosed what problem you are trying to solve.

To repair pitting either the base has to be machined, sanded, scrapped down past the bottom of the pits OR the base has to be built up and the pits are then filled in (and the base can be remachined, sanded, scrapped to restore the original dimension).

Examples of the build up method are brazing a layer of filler (brass, silver or even solder) over the pitted area or even brazing a piece of sheet/tube over the bad section. The laying down of brass is used when restoring damaged BB or steerer threads, followed by chasing/tapping the threads.

One challenge to repair rust caused pitting is that the pits need to be clean before brazing the filler over them. This cleaning often will further remove base metal. When involving a thin wall tube the resulting wall thickness can get so thin that structural integrity suffers.

I replaced a top tube of a 20+ year old steel frame this last Spring which had been sent out for painting and their prep revealed that some pits had come through the wall to become pin holes. the painter refused to continue with the paint job (as they should have) until the tube had been repaired. In this case it meant replacing the TT. As the TT was the only tube which was horizontal and could trap moisture in it (which collected along the bottom and could be seen as a "strip' of corrosion down the length once the tube was cut out) there were no other spots of complete rust through. Some remaining pits were filled with silver and sanded smooth for cosmetic reasons.

As cb asked, knowing more detail and the expectations would help us give more focused advise. Andy.

I've observed Chro-moly frames that have lived or been left out in the elements for years and acquire only surface rusting that is easily steel-wooled away. Chro-moly is apparently quite resistant to deep or extensive rusting, --I guess. I applied frame-saver inside the first used frame I acquired, but have never done it again. My commuter basically lives outside all day every day and still has only surface rust in a few spots. If I ever bother to clean it up, the rust seems to easily wipe away with mild abrasive. I guess I'm recommending a mild abrasive cleaning and a durable coating if rust bothers you.

I will take issue with dlb's assumptions. I have a basement full of cro mo with significant rust. I also have had the displeasure to service MANY steel frames which have seized BBs and seat posts and/or rust throughs and structurally affected fittings.

I do agree (even though dlb didn't really say this point) that with regular use and minor care the steel frame can have a very slow growth of rust. Many production steel bikes get a phosphate rust inhibitor soak prior to primer or paint. Even this simple act does a good job at slowing down rust. The wiping off of the steel with a rag periodically also does a good job. Any wax or oil film left behind further helps.

When I receive raw frame tubes (usually True Temper) I clean them, measure them (including the butt locations) then coat them with LPS 3. I have tubes that are many years old that have no rust on them. However the practice joints that I do each fall (prior to that Winter's frame project) are left unfinished/protected (but with the flux removed) and by the following Spring they show significant rust just sitting on my basement floor. Andy.

...another bubble burst. I was hoping that cro-mo really was impervious. Good info.