I had horrible luck with it on one gun.
I had a badly rusted cap and ball revolver.
I took it down to bare metal w/wet/dry sandpaper, then boiled it in hot water/detergent, then degreased it with Xylene.
The bluing came out all blotchy. It was flat in some areas, and glossy in others and in still others it was hard to tell there was anything done.
I used it on some pieces/parts of an old .22 rifle I was redoing and it worked fine there.
Glad it worked out for you!