Once the background is painted, the main subject can be painted. I lay frisket film over the whole board again, this time cutting out the shapes in the subject that I want to work on. (For example, when you paint the black areas in the interior, fenders, tires, etc. you would only cut and remove the frisket from those areas. ) It is better to make all your cuts in the frisket before you start painting, as overspray will be hard to see through to cut your other lines. Cut out only those areas of a certain color you want to spray. Work dark to light, not letting your dark areas get too dark when first applied, as they will get darker as you spray the lighter areas. You can reapply the cut frisket shapes and paint other areas, just be really careful to reapply it exactly where the film went or you can end up with white areas. When you get to a point where it it not possible to reapply the film, remove all the frisket and put another layer on the board, recutting any areas that still need paint. Remember to use the Workable Matt Fixative between sessions of applying frisket so that it doesn't remove any paint from areas you've already finished.
As the last step, remove all frisket and freehand airbrush any areas that still need color. You can use colored pencils or illustrating pens to fill in any tiny areas that the paint is missing, or to do fine detailing.
For frisket film and other art supplies, I recommend MisterArt.com, as their prices are very reasonable.