... Not sure what product is best to brush/wipe over the retainers that is smoothed in after the retainers are installed....in case anyone has a recommendation?
I missed seeing your request for a product recommendation.
I tried several products (caulking) that shrunk too much when laying it in as thick as what is required to conceal the expanded rivets. My solution was to re-coat a few times, each recoat was after the previous coating had cured (and shrunk). Eventually, it leveled over the rivets (I estimate that it was 3 or 4 coats). I hope you found a better product (since has been months since your request).
On Edit: I added an image of how thick the original sealant was. It is easily about 1/4" to 3/6" thick but not higher than the drip rail is deep. Notice that the sealant tapers down away from the drip rail edge.
Another note, I believe the sealant would be best added while the drip rail molding is still off the drip rail.