Preparation is key. If the model is plastic apply mild soapy water to get rid of any residue, if metal, a light sanding should be applied. Then, after allowing to dry thoroughly, apply grey primer. Final top coats would depend on what it is and what effect you want, so matte for worn-in fabric or leather, etc, jackets, trousers, hats, and so on; gloss for new or shiny leather, rubber, plastic and metallic for things like helmets, equipment, etc. Scale or size is a factor as well. The smaller it is the trickier it gets.