Wondering if anyone have any tips here. Often times I only want to untrim a small portion of the surface but the entire surface untrims.

My fastish work around for this is to thicken the desired surface, remove face the areas I only want untrimmed, offset the face I want to "0", hide that surface and delete the solid.

That method works best for me but another method is to make a copy of that surface, convert to curves and then untrim the surface, then modify curves to the shape I want and either use the curves to split the surface back up or extrude them and use the extruded surface to split the surface back up and delete.

Also, is there a way to trim a surface faster then splitting and deleting the portions you don't want ?