I am asked this question quite often. To many, the answer is surprisingly – NO! If bluing is just worn and there is no apparent active red rust, Rustblue can be applied directly over it and will blend the old with the new and render a uniform texture. It is not necessary to remove all existing blue. I have restored many firearms blued by the rust blue method and by caustic bluing or the hot water method. Rustblue always results in an undetectable restoration of the finish. Why? That’s because the blued surface formed by these methods is of the same chemical composition. Rustblue creates a matte finish due to the atmospheric rusting process, whilst the other methods develop a more polished finish. Rustblue is far more tenacious and deeper. Blues formed by cold blue solutions are not the same and need to be completely removed.
Active rust definitely needs to be removed. But unless the entire metal surface is involved, only those areas affected need to be taken down to bare metal or neutralized with rust removers. Just make sure the finish texture matches the unaffected areas. These stripped areas can be treated with Rustblue solution and converted until their shade is close to surrounding unaffected areas. Then, uniform coats of Rustblue can be applied and blend the entire affair together.
Good luck in your bluing!