Invalid surfaces being created when joining

I am joining a model and I seem to be creating invalid surfaces as I join. Sometimes if I join each part incrementally it will work but I am hitting a wall on this one. The invalid surface moves around and surfaces will flip. I will upload file of surface post join invalid. Any advice as to why. ThanksBadObjects.3dm (66.2 KB)

Mike,
I just opened your download and there is nothing there, you need to re-post the drawing.

All my best … Danny

Mike,

The file you uploaded has a single invalid trimmed surface. I don’t know why, but something about the trim is bad. If you DupBorder, then untrim the surface, and then split with the border curve, you get a valid surface. Will it then join your other surfaces?

DupBorder Untrim was a great fix. Thanks