Make a surface out of two 3d lines?

i try to make a nice surface between the two lines you
can see in the file.

i try it with patch an loft but the result is bad.
i need only fill the exact space between the two lines is
there a nice solution for this problem ?

thanks a lot

surface out of 2 3d lines.3dm(396.4 KB)

A good way of making a surface between two curves instead of Lofting or maybe sweeping would be to extrude them both a bit and then BlendSrf between them with G0 continuity, that way you can fix areas where the surface is folding back on itself funny. The other solution, and what will probably give the cleanest possible solution given the input curves(which are not that clean,) is to Explode the curves and tackle the loft piece by piece.

Create a straight line between the 2 polycurves at both ends.
Polycurves are the rails.
Added lines are the sections.
Use the “Add Slash” feature to control the shape. Adding “slashes” changes the orientation of the isocureves on the surface between the rails.

Example surface created using Sweep2 command and Add Slash feature.

surface out of 2 3d lines DC.3dm(527.1 KB)

great thanks ! so glad !