thermoset finishes require abrasive blasting before application. I use Aluminum Oxide grit when blasting as it leaves a smooth even surface. I also use Garnet sand but only if they are out of Aluminum Oxide. The blasting process removes all of the rust and blends the pitting (if any) with the metal around it. If coupled with a Matte finish and the pitting/rust fairly minor youd be hard pressed to locate the spot the rust was in before. This is the reason i most like thermoset finishes.
Secondly, cerakote, the one i use most, is weather proof. It is a 2 part coating, not all that different from epoxy and once applied to a bare metal surface with the proper blast profile will bond with the metal on a chemical level and seal it. So for ever more you can use your shotgun or whatever in a down pour and the surface wont rust up as a result.
The 4 shotguns I have on schedule for this weekend are all just as you describe. an 870, 2 1100s and a beretta 12 ga auto. All were left in shotgun cases that were damp from the morning dove hunt. the result is spot rust all over. The guy initially called me inquiring about a reblue for all 4, but after speaking on the phone a bit and then showing him a few examples of the finish applied to some of my own firearms hes decided to go with a semi gloss graphite black finish. it will be very close to the original bluing only much tougher and rust proof. They use these shotghuns for trap and dove hunting. so they will see lots of future use and do all exhibit evidence of being fielded. They are all fairly dinged up.
I dont have any personal experience with duracoat. ive never used it. I have seen examples of it and it is very similar to Cerakote, but I dont think its quite as durable.
I apply cerakote with a gravity fed HVLP gun equipped with a .8 nozzle. Which is just a step above an airbrush. so the coating is very even and uniform.
It takes 43 muscles to frown, 17 to smile, and 3 for proper trigger squeeze.
The latest caliber or gear is no substitute for experience and skill. Rifles and cartridges don't make hits -- shooters do.
Fact of life:
After Monday and Tuesday, even the calendar says WTF!