Softboxes reduce light output, sometimes by half depending on the box, so you would need to compensate. Most professionals that light paint simply use an Ice Light, or a homemade light made from a long fluorescent bulb and a battery inverter. The larger the light source, the larger your reflection will be. And remember, larger light source = softer light. So adding a soft box to a small light will not "soften" per se, only just modify the light a bit.
Light painting cars is essentially a way to imitate photographing a car in a studio with a huge white surround and ceiling. It can be as complicated as you want it to be, or as simple.
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