All the sheet metal rebuilds and repairs and re-skinning that I do get etch, alodine, and epoxy primer on all the surfaces.....that includes under skin laps and under rivet heads. Of course, I'm talking about new parts and parts that are completely unriveted from the airframe.
I can understand the idea that etch not getting rinsed out from under skin laps could be a problem, stripper could also be part of the problem. Good old zinc-chromate in the rattle can has done a fine job for alot of years on many of the aircraft in service, and when appplied to a clean surface (not necessarily an etched surface) will stay on for, what, 40 years?.
I believe that the zinc orthophosphate in the DP series of epoxy primers from PPG provides the "sacrificial' element that makes the DP series the choice of many industry professionals these days, myself included.
Some of the guys around here that once utilized so-called "self-etching" primers have ceased to apply them. I can't give any concrete reasons why.
DAVE