If you watch any videos about Formula one machining, or facility's they always show a gantry mill(cnc router) machining a wood/composite material that is tan in color. they make molds for carbon from it and parts for wind tunnel testing. The red bull X1 prototype is made of this.

WHAT MATERIAL IS IT

 

It's called a high density foam. One type is Renshape and you can find it here:

http://www.freemansupply.com/RenShape5045Polyur.htm

I've used it before for making carbon molds and it takes very little post processing to achieve a nice finish. Some light sanding, primer filler, then a good waxing. Hope this helps.