This is how I do it:
For scratches:
Use dark metallic grey base coat (possibly enamel) and use the desired color for top coat of the tank (preferably acrylic). Use cottom swab dipped in alcohol or acrylic thinner and rub against the acrylic paint. Some paint will be removed exposing the metallic paint beneath.
Or:
After applying the top coat, get a stiff and entangled brush. Dip in a dark metallic colored paint. Brush on a cloth to remove most of the paint. Brush against the surface of the model in whatever pattern you wish to simulate scratches or chipped paint.
Then apply a weathering wash and if desired some rust colored paint on the exposed portions.
For chipped paint:
Use dark metallic grey base coat (possible enamel). Use liquid paper to mask certain areas or spots you would want to expose. Spray desired color for top coat of the tank (preferably acrylic). When the topcoat is dry, rub off the liquid paper to expose the metallic paint beneath.
Expect other suggestions from other members to follow.
Good luck.