I am just learning and doing my first bit of rust bluing - some scope rings that I have fitted to existing claw bases. They have come out very well.

Now I have a couple of guns where the bluing has worn thin in places, and another that is scratched. Can you rust blue to touch up, fill in and darken thin patches or do you have to take it back to bare metal. Reason for not wanting to take right back is you loose a lot of patina etc

Yes you can touch up areas. I use the acid rust blue and use a 1X3X6 inch long of aluminum block to create steam by heating the block very hot and use a cloth with water to make steam on the spot to rust blue. Make sure that the area is free of oil before you start the process. Use a wire wheel or steel wool to card off the rust. You will need to apply 4-5 coats before you get the colors you are wanting. I use this style on O/U shotguns and doubles when a small area needs to be treated. Every thing has to be clean and no oil on parts or steel wool.

Yup, what Les said. Clean, clean, clean, no oil! Use steam or boil.

Greg