I have been using self etching primer on steel parts as well as aluminum parts, but have doubts it is beneficial on steel parts other than to temporarily prevent rusting.
I painted over the original paint on cast iron 10E castings.
Cleaning up aluminum parts is tricky. Too aggressive and erosion occurs. As for polishing, the oxide polished better than the bare aluminum. Once ya break through the oxide ya have two tone coloration.
I have resolved myself to accept 'better' as a goal. I am far less aggressive with aluminum cleaning than I used to be.
Beware oil/grease/ whatever contamination especially with aluminum parts.