I have had very good experience with doing this when i know the same rust blue was used originally. And i have not had good luck with mystery rust blues as the original process.

You wont know until you try. The best way is to prep the area needing done and then run a number of cycles on just the spot to get it to catch up tho the surounding bluing. Then do a few overall applications to even things out. By the time you are into it, you will know if it is working or not.

B.Dudley