I deburred, dimpled, prepped and primed all of the elevator stiffeners along with the spar reinforcement plates and trim access reinforcement plate (both in the foreground).
After the primer dried (15 minutes or so), I installed the platenuts on the trim access reinforcement plate. Countersinking the reinforcement plate was definitely the right way to go. The mounting surface for the trim access cover is totally flush.
Here is the backside showing the platenuts and rivet shopheads.
