Thank you Adrian, Nap, Alex and Mike for you comments!
Adrian, I really wanted to do the helmet as realistic as I could. I don't really know why, but I wanted to paint a German helmet for quite some time. Maybe it's because it's such an iconic (I think I could say iconic) piece of gear, and I wanted to give it justice.
So, I googled german helmets photos to see the real deal and tried to reproduced it as best as I could.
I noticed that an used helment as lots of dots and scratches on the painting and the surface isn't that smooth.
From here, I mix a colour that looked nice enough for me, painted some lights on some areas and the with a spounge with a lighter tone I started to add some dots and scratches on the surface. Then, some dirt, rust and some washes of different tones of Brown. I also freehanded painted the simbols.
On the the edges of the helmet I used metalic paint. I noticed that this areas loose the paint very quickly and do not get very rusty because they are constantly being polished by the the contact with objects and hands.
The colours I used were german gray with light blue for the helmet colour. Then different tones of brown and english uniform.
Hope it helped
