What were you using as the light source? Looks fairly concentrated and hasn't made much, if any effect on the bodywork. Personally wouldn't bother light painting a light-coloured car, speedlights/strobes are probably a better way of fully illuminating them and getting the white to pop. If you do want to lightpaint though, here is what I did for this shot (admittedly a black car not a light coloured one..)
Equipment:
1x Bunnings Bat-light
Linky (I have two of them, and they are amazing, you can either run one bank of LED's or two for extra power in really dark situations)
1x Foldable soft box (Used this to diffuse the strong LED lights and create a larger reflection on the black body. For a lighter car you can get away with more direct light cost ~$30 from eBay and folds up real small)
Can't remember the exact EXIF but something along the lines of 12 sec, f8, ISO 50.
Location: The darker the better really, for the car, the background is fine to be lit, but the car should be in almost total darkness for best results IMO.
Process:
Concentrate on one panel/side of the car at a time and combine them together in PS. After practice I can light a car like this in around 10mins including change of exposures for headlights, tail lights and interior so it's fairly quick and simple
The key is making sure your "sweeps" with the light source are as smooth and flowing as possible. Remember about reflection angles to the camera, and how these will effect where you need to walk to get the light trail in the right spot.
1. Side panels- To accentuate the curves in the bodywork, point the light down towards the floor and keep it raised above the roofline and do your walk across. Remember to walk slightly around the back and front of the car to ensure the reflection encompasses the entire body.
2. Wheels - Point the light at a lower angle at the wheels, and walk around the lower portion of the side panels.
3. Roof/bonnet/windscreens - Create a nice rim light by walking around to the far side of the car (side furthest from the camera), raise the light and point at the roof. Walk along the full length of the car as this will also pull out the front and rear windows, and also the bonnet. In my example above this technique lit up the under side of the spoiler too which was a bonus!
Photoshop processing:
Bring all the images into the same .PSD. Change the blending mode on each of the layers from "Normal" to "Lighten". Instantly you will see the car light up, but you will probably have white lines in the sky above the car unless you were really careful. Remove the light traces in the sky by selecting each layer and masking them out/erasing. Remove any odd reflections between layers that you don't like. All up it took me only 15min to process the first draft of the example above however you can spend hours perfecting it if you desire. My final images tend to take around 2-4 hours of colour grading, cloning, sharpening and general tweeking.