My first ever attempt at a turned aluminium cowling effect....
Base colour is Tamiya XF-16 Flat Aluminium with a little light grey added to dull it down. When dry, I made the swirls with an acrylic paint pen I sourced from an art supplies shop. Utilises a ball type pen nib, flows freely and easily controlled. Much easier than using a paint brush.
I expect you'd need a finer pen (if available?) for anything smaller than 1/32 scale.