Create a new surface quilt to prepare the model for turning into a protrusion (water tight surface geometry). Join the surfaces, intersect will not work. View – Advanced – Mesh Surface - Quilt will help you check to see if you have joined all of the surfaces into one singular quilt.

Create two new surfaces (on the centerline and on the bottom) to be used to cap the open geometry.

It may be necessary to create one surface as a flat - sketched (using existing edges of the car) if you cannot get the Merge – Intersect function to properly work.

If after all this, there is still one edge that shows yellow (one sided) you will not be able to turn this into a solid. An example is shown below where the black line at the rear corner is a one sided edge. Checking the surface tangencies shows that the surface is actually tangent to the body panel at this point, but for some reason the curves must not perfectly match up.

One of the ways around this problem is to reset the accuracy. For this particular problem, the accuracy (Setup – Accuracy) was changed from .0012 to .0018 and this value was found by trial and error. Another way would be to iges out the surfaces, start a new part file and iges in the data. You would have to do a Redefine on the imported geometry (assuming that it iges in with the same problem) and do a Heal Geometry – Automatic etc. Of course at this point you have lost any associativity with the parent geometry.

After turning the quilt into a solid (Create – Protrusion – Use Quilt), it is necessary to mirror the solid. Because there are a million references, it is probably easiest to Mirror Geom. Any colors will have to be set on the mirrored geometry as shown below.

The last thing to do is to cut out the wheel wells. Using your imported datum points for the wheel well locations, create cuts all the way across the model. It will probably be next to impossible to shell out the model due to the different continually changing radius of the surfaces. Offsetting surfaces (normal) will have equal problems. Leave the model solid.