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.