There is one thing I don’t understand.
When painting metal doesn’t the metal first need a primer coat for the paint to stick on to?
Otherwise wouldn’t the paint just flake off?
I don't normally use a primer on GHQ pewter models. Some others do - it's rather up to you. I generally soak the model in soapy water overnight to make sure that there aren't any greasy fingerprints from assembly. then I use an airbrush to apply the base coat color - and normally use acrylic paints.
Some use a primer - and that is fine if you like it. Just be careful to not apply so much paint that you cover up the fine details.
I don't use a primer either. I have found that the pewter is grabby enough to hold the paint, and I think that the less paint that you can use, the less you cover up some of the incredible cast details!