Skin mapping
We see our skin as a flat 2D texture in jpg format, but in LFS the jpg is literally wrapped around the 3D model. Obviously it would not be good if the skin covered the glass areas and some other parts like wheels and brakes. So not all of that JPG texture will be seen in the game.
If you download and unpack the LFS CMX Viewer you will notice a folder called map_info. In this folder you will find JPG files that clearly show the skinnable areas highlighted in blue. Some other areas are shown in green, usually an exhaust or other "component" like the rollcage or mirror stems. The grey areas not highlighted will not appear in game, no matter how you paint them. Don't be worried about getting paint on the wheels, tyres, glass, door locks, bonnet pins, grilles, brakes, lights, diffusers and other non-skinnable areas.