I'm not the biggest fan of floating props in vacuums, though this is quite common in Design Studios. If you can, painting your props in full scenes is always helpful to sell the context in which it finds itself in. Continuing on the technical nature of wheeled vehicles, there is also another video here going through Photoshop's paths tools. They can be quite tedious, but once setup, they offer quite a bit of flexibility with changes and adjustments that need to be done later once the painting has moved pretty far along. This isn't generally how I prefer to paint, but for those clients that like clean and tight design work, these techniques are pretty useful.