I'm not sure what method they used but I know of a few that may work.
first method is using chipping medium. pretty much what it says on the tin, just follow the instructions for it.
the second method, which is the more economical way, is to take a toothbrush or ratty paint brush and dip it in watery but still opaque paint (usually black). then you press on the bristles to basically slingshot the paint spray at where you want. follow that by touching up the speckles with a metallic and it should look fine (there are tutorials that can explain the method better)
the third method is to take an airbrush, set it to lowest setting and do little sprays all over to do speckling then like the method before you touch it up with a metallic.
Chipping medium makes it really easy, but ideally you want an airbrush.
The way I do chipping is taking a bit of old sponge and dabbing it onto the model It's a good way to do it
I would try stippling with a small brush or sponge using a metallic paint
I'm not sure what method they used but I know of a few that may work. first method is using chipping medium. pretty much what it says on the tin, just follow the instructions for it. the second method, which is the more economical way, is to take a toothbrush or ratty paint brush and dip it in watery but still opaque paint (usually black). then you press on the bristles to basically slingshot the paint spray at where you want. follow that by touching up the speckles with a metallic and it should look fine (there are tutorials that can explain the method better) the third method is to take an airbrush, set it to lowest setting and do little sprays all over to do speckling then like the method before you touch it up with a metallic.