How did I smooth the edges of a posterized image?

This is what I did years ago:

This is all I've managed since yesterday:

Seems I have forgotten how I did this a couple years back. I'm using PP. I can't smooth those edges as much as I need to.

I thought all I had to do was repeatedly apply the Remove Noise effect, up to a dozen times or so. Whatever I did, I clearly remember that I saw the edges smooth out a bit more every time I applied the effect. EVERY TIME. I would stop when I thought the edges smooth enough for a kid to trace accurately.

Now, the edges smooth a bit the first couple times I apply the effect, but I reach a point at which the effect no longer works and these edges do not smooth any more. The smallest floating blobs do not disappear the way they did before. No normal kid could trace shapes as complex as those in the second image.


I can do something similar by using the Smooth effect several times, and then applying the Posterize transformation again, but I really do not remember doing it that way before. I don't remember a blurriness being part of the process. I could swear all I did was Remove Noise repeatedly, but maybe I'm not remembering clearly. It has been three years.

If you could reverse engineer what I did three years ago, I'd greatly appreciate it.