I'm thinking you're talking about the inside surface of the exterior end cap? That's what has the sealant on it and I'm not aware of anyone who has used that as an interior surface.
Older trailers like my 57 have both interior and exterior aluminum end caps with insulation between them and are often painted with Zolatone. These can be stripped and polished.
You could fabricate a new aluminum end cap and make the same effect.
Hope I'm reading your question right.