Complete shape.

Finally, we shall create a complete shape out of car's half. First of all, reset object's axes to world, using Display\Local Axes\Reset to World tool (in "offset" mode). Then click on an object with Create\Copy tool selected. Once copy is created, rename it in objects browser (to "copy" or "Surface2" or whatever else).

Pick a Modify\Mirror tool. Make sure "Mirror geometry" is on, "Base to Pivot" is off. Switch to World-X mode and click on an object (or on it's copy - doesn't matter). One half will be mirrored:

On the image, original object was mirrored. Notice that it's flipped inside out. So, switch it to polygons level, switch to selected mode, select all polygons and apply Modify\Flip tool. Now it should look fine. Also notice that copied object ("copy") isn't "flat-shaded".

Using a Modify\Attach tool, attach "copy" to original. Now you have a single flat-shaded object, but it's not enough. Make sure to apply Surface\Normals\Calculate tool to the object. We shall use weld tool which requires properly-calculated normals to work correctly. Calculate tool will do it for us.

Switch to vertices level and zoom closer where two halfs comes one to another. They are not welded together on this line:

With a Select\Open Edges tool, right-click on one of such a vertices (vertex should belong to one of these non-welded set of edges). A sequence of vertices will be selected. Highlight one vertex from such an "open edges set" of another half. You can use -hit technique to highlight such a vertex:

Apply Select\Open Edges tool to this vertex too. You have two rows of selected vertices. Invert selection and hide all the stuff except these rows. Then select all remained vertices and switch to selected mode. Image below shows what should remain and how it should look:

Notice that each vertex shown is actually a pair of non-welded vertices and a pair of normals drawn in each point. This is what we need to get rid of. We need single vertex with it's single normal instead each pair.

Pick a Modify\Submesh\Weld tool. Expand it's options box and enable Multiple-target mode. We have vertices selected, and values in weld options box are set to default. In most of cases, these settings are quite fine, if you were accurate in modelling. You can click in the view and weld tool will do it's trick: each pair will turn to single vertex. But it's a rare case. I assume you were not that accurate and (like me) still have some non-welded vertices:

I've drawn orange circles around two pairs of vertices that are still not welded. Of cause, I can set bigger Precision value in options box and weld tool will of cause weld these pairs. But it might (and I'm sure will) weld vertices that I've surrounded with blue color - these vertices are also very close to each others but don't need to be welded. In such a case, switch selected mode off and hide the vertices that might be welded by mistake. Not all vertices should be hidden - only vertices that are very close to each others and might be welded by mistake. I personally do it the following way: