Hi Pilou - for doing a loft you don't have to draw a line, once you have the original curve and a copy above it, you select those 2 curves and do a loft to get the surface. You'd need to draw a line if you were using something like Sweep instead of Loft.

Any way that gets the job done is fine of course! But using Boolean Union in that way will rely on the Booleans detecting all the exactly overlapping faces between all the pieces, which should work but it will be doing a lot of analysis and detection work in the boolean to figure that out.

Doing a Loft + Join ends up doing a lot less internal calculations, and just in general a method with less calculations is good because less calculations = less potential for things to go wrong...

- Michael