Because your shape is spherical, I wonder if the 3DP surface could be improved using a dedicated sanding block tool of suitable size, which has a matching cup profile dedicated to that shape? It would be difficult to get a 2D abrasive sheet to conform to a full inside domed profile, but I was thinking more like an outer, annular ring of rubber sheet backing + abrasive. Say 1" perimeter ring of abrasive for an 8" diameter sphere the inner part does not contact. But you model the contact area truly spherical but compensating for the thickness of foam rubber sheet backing + abrasive paper sandwich. For example, if you've ever seen how balls are made, that kind of principle.
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