Now, cleaning these parts is not difficult, I process them with carburetor parts I restore, the exact method is proprietary, however for this job DYI is easy. First use a good degreaser like Purple Stuff or Simple Green to get the grease and oil off, then a soak in a store bought corrosion remover like Ospho, or a home brewed mix of citric acid, vinegar, and salt (as described by another member matthewnokc) also does a good job. His recipe: 1/2 gallon of white vinegar- 1/3 cup of salt - and the entire contents of 1 of those yellow plastic squeeze lemon juice bottles.

Soak the steel housing and the brass plunger in the solution overnight for the best results, clean off residue with a clean shop rag, use a scotch bright or 000 steel wool if there are stubborn stains. The original plating will be removed if you use the home brew vingar/citric acid mix above so the housing will now be bare steel (my example was also stripped of the plating and is bare steel as shown here).

You can use damp baking soda and a cloth to polish the surfaces, the baking soda works especially well on the brass plunger as a polish and leaves an OE appearance rather than an overly polished high luster that polished brass can have. Use dry baking soda in a cloth to buff it up to a shine.

Once you are happy with the housing finish you can use it as is, give it a light clear coat with something like Eastwoods “Nylac”, or do what I do and send it out for a proper silver/zinc plating job.

The example here is not plated yet, I just assembled it for show and tell, I will be taking it back apart send it out for final plating.