How to remove rust and restore 57 marker light receptacle?

Chiming in. Have the same marker lights on my Westerner. Soaked mine in a vinegar/water solution for 24+ hours. Helped a little at loosening the rust. To really attack the rust, I drilled a hole in my plywood top work bench, bigger than the back of the bulb assembly. I removed the wiring pigtail/spring, then inserted the bulb housing into the hole making the front of the assembly flush to the top of the workbench; then secured with screws. Using a crimped cup brush attachment and drill, you can then polish the inside of the marker light. I used a flat blade screwdriver to remove any dried up gasket prior to polishing. To clean the back side, I mounted the assembly face down on the work bench, then used the drill/brush to remove the rust. I used a battery post/terminal brush to clean the inside of the bulb socket.  New gaskets can be purchased to complete the restore. Before installing, I placed a circle cutout of aluminum foil in the assembly to increase the brightness of the light.

My question to others, what can be done to the metal housing to prevent future rusting, besides Rustoleum? A clear acrylic spray?

I would say it weakens it? The wire pig tail and spring should pop out. Might need to use a nail or something on the back side to coax it out. I also used diaelctric grease to coat the pig tail end before re-installing to prevent rust from hindering the bulb contact.