Hi Graham, ok now I understand.

Your problem here is your extrude is not a solid, it only has the side walls on it. You need it to be a fully enclosed solid with caps closing off the bottom and top.

Normally you would get caps put on automatically (unless you turn it off by unchecking the button for it), but that can only happen if the curves you are extruding form a fully closed loop.

In this case you've got a slight opening on both the outside and inside outlines at these spots:





See how the ends don't line up with each other on the zoomed-in view? You need to fix up those ends so that they touch each other, either by trimming or by using Edit/Show pts and moving the end points around. There are 2 spots like this to fix up.

Once you do that, you will be able to select both the inside and outside curves, and then do an extrude, and it will generate a solid that has sealed caps on the top and bottom - that's what you want.

In this case you can also skip the sweep step, because your sweep in this case is made up of 2 lines which will make a plane - the extrude with a fully closed outline will already make a plane at the top. You only really need to do the sweep step if you want a more textured or curved piece on the top instead of a flat plane.

Let me know if you have any problems with making this next step work!

- Michael