Mine is quite rusty, so wondering if the steering shaft itself (solid shaft that goes into the coupler) is painted black like the remainder of the assembly?
If not, what have you used that holds up? RPM may not last long on my application as the shaft is pretty darn close to the exhaust manifold on a BB A-Body. I was thinking a paint like cast blast, which may not be correct in color, but will hold up to the temp.

Soak the shaft in EvapoRust. If it was painted black originally, you'll see black paint once it comes out of the dip. Even extremely rusted pieces probably have some hint of paint still on them.(under the rust) You'll be able to see it if you chemically remove the rust instead of blast it off.

I'm ahead of you. Been soaking in Evapo since Sunday, will check tomorrow. Didn't look too promising for seeing paint though.

Here is the a pic of the original steering shaft on my 69 Dart.

I don't think it is painted. I think they were plated black phosphate and covered in a light machine oil. I would plate it and then put RPM on it.

I never had one that was painted from the factory.

Shaft is not painted. This is after soaking in evaporust. At least now I can get the bearing off. Quite pitted. Need to do some work.