Hi Peter, here is the results with just a bit of quick tuning:



I joined your longer curves together, they were already matching up smoothly (looks like they were split from a single smooth original curve).

I also joined together the 2 short pieces and then turned on control points and used Transform/Align to make the 3 points around the segment join location to all be horizontal with each other so those segments were nice and smooth.

Sometimes it can also be nice to just delete the control point where 2 segments touch to fuse them into one single smooth segment.

Additionally when using align I picked location that used intersection snap to move them to be all lined up with the other curve instead of having a space between the curves there.

Then you can select all 4 of these curves and run one single Network command to create the surface I show above. These curves are attached here as car1_tweaked_curves.zip

Whenever you have a curve network that has curves with sharp creases in them, that will definitely cause sharp creases in your resulting surfaces as well.

Hope this helps, let me know when you run across other problem areas and I can see if I can give you some tips for those as well.

- Michael