I typically use Zolatone paint on the undersides of my builds. I don't know about the originality of this paint on a GT40 though if that's important to you. I build my cars to be "drivers" so the finishes need to be able to stand the test of time and miles. Zolatone looks good (maybe not show car great, but good) when first applied. It will most likely look just as good 20 years and many, many miles later. It's used on the inside of boat hulls (so stands up in marine environments) and hides dust, dirt, and grease well.
