I had my box rebuilt and noticed after that they'd sprayed it - i looked good for about a week, then it got dirty, the paint was cracked and flaking within 2 months and it looked worse than before it was painted
dunno what paint they used but there was loads of wheel spray about the place and they only do gearboxes?
i suppose it might be worth cleaning if its gonna be a show car - if its a "proper" car i wouldn't bother, it'll only get covered in sh!te again