Hi Peter, I'm glad that you are getting some better results now!

Here I'll try to describe why doing things in larger pieces makes smoother results.

Imagine you have 4 points like this:



Now imagine that you wanted to plot a curve that passes through those 4 points. Such a curve might look like this:




That's a very natural shape for just those 4 points. However, in a certain sense the shape ends very abruptly at the end point and does not have any "knowledge" of any kind of a shape beyond those end points.

So if you mirror that shape, you can see that the mirrored result is not smooth as shown here:



In a sense the mirroring is an attempt to continue the shape, but the shape itself came to an abrupt end and did not know anything about a continuation.

If instead of doing this, you take a larger point set, like this:



Then it is possible to plot a curve going through all those points, like this:




Now you can see that the curve does not have an "abrupt end" at that center axis line, it is able to see how the shape continues past that and this will create a smooth shape at that axis line instead:



This same thing comes into play when building a surface out of small pieces. A small surface piece that gets mirrored later is very similar to that first example above. Unless it is built to carefully have a mirrorable shape, it is easy for the surface to just have an abrupt stopping point and end up with a crease when it is mirrored.

Anyway, I thought I would try to explain the reason why doing things in a larger chain can help to give smoother results.

- Michael