Several books cover this and you may find a few things in ancient Metal Finishing and AESF magazine. If you have a real problem, try using thieves. This is simpler but wastes nickel. Nickel is cheaper than your sanding time tho. The shields require some very good guessing, or blind dumb luck or grace skill and cunning.
Beginners are frequently short of that, but experiment.
It will make you a better plater in the long run. I cannot help you with the problem. I am a former Hesston farm machinery dealer, and have collected a number of your companies buckles ect. If so please forward. All information presented here is solely for general reference and does not represent a professional opinion nor the policy of an author's employer.