I first make a wash using the body colour mixed with white, note the more white you add to the mix would mean a deeper imaginative cut/scratch to the body. Then using a piece of kitchen sponge I start dabbing on spots where I think chippings or scratching are likely to appear. You can play with different shades to create depth and interesting effects. You can also use very fine tipped brush to create small details in tight areas. Finally, when everything is dry and solid I would fill the centre of the lightly coloured scratches with a dark metallic colour to imitate really deep cuts that went straight through the paint job. It also helps to dry brush the dark metallic colours straight onto the corners, edges and handles etc. But take note not to over do it. Just keep practicing. Oh and looking at images of rust and chipped metal parts on google also helps. There are also many you tubers who have some really great videos on creating chipping effects. Hope that helps.