Hi mir4ea, it's working ok with surfaces for me over here. Can you please post a surface that is not working for you?

One thing to note is that if the outer boundary of the surface is already the natural boundary of the "underlying surface" then you won't see anything happen during the untrim, it's only when the outer boundary actually has cut away some material from the underlying surface that you will see anything happen.

You can get an idea of what the underlying surface looks like by turning on the surface control points.

- Michael