For Watt it's worth....
At
?t=730 this video looks at the underbody of the Ford Lighning. I thought the bellypan was interesting. It's basically a full length skid plate with massive corrugations.
The point being there will never be laminar flow in the underbody. It's an open-sided plenum with one moving wall. The ground or the vehicle depending on your point of view.
Suggesting that continuity of surface is more important than smoothness.