I have done further trials with my Vallejo Old Gold paint mixed 2/3 -1/3 with Vallejo Natural Steel then let down for spraying with Johnsons Klear.

In the first two photos I used the same brass safety valve cover as a comparison piece but realised after taking the photos that the brass had tarnished slightly in the intervening time since I did the last comparison.

I used an image editor (Picasa 3) to add some contrast to the same image.

Then I took another photo after polishing up the brass valve cover again so the comparison is more like for like than the first photo.

And again with a some contrast added

I have reached the conclusion that it is possible with good preparation of the whitemetal casting to make a decent fist of painting it with brass paint as long as there is no actual brass in close proximity to allow a direct comparison. I thought that I would pain the dome too to see what a difference a bigger subject made and while it looks okay to use it in practice I would need to fill some holes in the rear of the casting that you cannot see in the images.