Not to high jack the thread ...but... I always understood that the spring assemblies were either all natural or fully assembled and dipped in black paint.

One of the things I always see on graveyard cars is they have black painted springs with natural clamps. That would also mean the inner liners would be natural not black.

I have disassembled spring assemblies and since they are 40 years old it is hard to see the paint, but they look like no paint in between the each spring leaf.

I have never powder coated springs because of the clamps.