Jem90th pickguards have been a problem since the model was introduced. They cloud very quickly and are difficult to near impossible to clean. I have had some good luck using a simple buffer and paint polish to remove light corrosion with just the lightest staining left when finished, but I recently had a heavily corroded guard that would not budge even using a bench polisher and black rouge. The guard brightened considerably but hidden by the corrosion was considerable pitting.
After polishing - a combination of pitting shown here, with staining in the following shots.
It was time to get to the bottom of it so I took it down to my local metal shop that makes my saddle shims for an evaluation. They concluded it was polished stainless and said that all they could do was try to continue polishing on their bench buffer [same as mine LOL]. So I started calling everybody in the phone book until somebody advised me about a metal finishing shop that wasn't in the book. I called them and they said bring it down. They concluded the same thing until one of the guys took it over to some serious arbor polisher [NOT like mine LOL] and did a cut on the back with course grit, to reveal, brass. They are either nickel plated [which would explain why they corrode so fast] or chrome on top of polished brass. Either way when the sweat and oils start leaching through the pores in the metal down to the brass it starts pitting, not much different that the chromed brass under your toilet you inadvertently "wet" on those blurry eyed mornings.
This was a full service metal finishing shop that did everything from plating to powder coating, and a specialty in refinishing marine parts. They chemically stripped the plating, cut the brass to the depth of the pitting, polish it, and then chrome plate. The cost was $100 and the results, while not absolutely flawless, were excellent and easily passable. I think you'd have to agree.
Although it sat in the case for 2 days and when I pulled it out there was already a thin "film" on it, probably from the case evaporatives still attacking it. You couldn't see the film unless you wiped the plate, which revealed perfect chrome and a very light edge to the "film". It will be interesting to know what it would look like after a few months in the case.