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You'd be surprised how strong polyester based fillers are under epoxy.
It sounds wrong in theory,
but in the real world it's a normal boatyard thing to do.

I'd define hidden spots as spots that pop up after prime that need to be smoothed out.
They could be anything from 60 grit swirls, low/high spots, bad feathering or whatever
would bug the heck if it was the final topcoat!
TP