How do you achieve this effect?

It can be overdone. So any advice you get here, use in moderation! My preferred method is to basecoat the model with the desired colour, perhaps nato black or red oxide, then use a product such as Maskol which dries to a rubbery consistency, applied with a small piece of scotchbrite. Then spray in the normal way, the Maskol can be then removed with a cotton bud dipped in Maskol and allowed to dry, or even a pencil eraser.

Vinnie

To echo what Vinnie wrote; go easy on any technique you choose to use. The vogue in paint chipping/wear right now is so over-the-top as to be garish and unrealistic.

I also recommend using Tamiya Acrylic XF-69 NATO Black for the wearing of paint. I apply it two ways; one with a #25/0 Brush and the other with bits of a Scotchbrite Pad, ripped and torn.

For imparting Silver/Steel, I've begun using Vallejo Acrylic Model Color #865 Oily Steel - which is plenty bright in 1:35th scale but works fine when toned down.

Gunnie

Best thing to do is check some reference photos of the real things to see just how much (or little) is seen on vehicles in use.

I agree completely with Jim and Vinnie that this is getting way over done as modelers use other models as reference and try to make theirs "more realistic" by adding more and more chipping.

It's like "mud" as some builders laddle on mud like demented cake decorator with an overabundance of frosting and one major sweeth tooth.

I use a salt technique, paint the base coat, paint the areas that you want to show metal siliver, wet salt and apply over this area, spray the camo, remove the salt. You end up with this...