So are the triangulated facets here actually BREP surfaces, then?
One option might be to just use
Delete Face (
Home tab,
Synchronous Modeling palette in the ribbon UI,
Insert --> Synchronous Modeling in the older pull-down menus).
If you want to create a clean edge near your red boundary, you might project a boundary curve and
Divide Face (
Home tab,
Feature palette,
More button,
Trim group, in the ribbon UI,
Insert --> Trim in the older pull-down menus) first, before deleting.
And if your inner panel is a solid with thickness, then you may want to project a boundary to both the inside and outside, divide both, delete unwanted faces from both (at which point you'll have an open sheet body), create a ruled surface between the new boundaries, and re-sew the solid closed.
Does that help?
Taylor Anderson
NX Product Manager, Knowledge Reuse and NX Design
Product Engineering Software
Siemens Product Lifecycle Management Software Inc.
(Phoenix, Arizona)