There's nothing wrong with wet sanding, other than the mess, lack of visibility until the surface is cleaned and the ease in which one can burn through the surface without realizing you're getting close. And the mess. I prefer to leave the wet for water sanding and polishing the final product.
However, polyester primer is a sponge. It will absorb water faster than any other product you can put on a car. If it doesn't adequately dry before another coat of whatever is put on top... well, you'll have a water-based finish unlike any other. Even a light dew in the morning can get sucked into polyester primer,and with the days getting shorter with less air dry time available, why push your luck?
Anything you can accomplish with water sanding, you can do just as easily and thoroughly with dry sanding, without the associated possible problems.
You can paint over polyester primer, but remember my sponge comment? It also applies to finger prints, film left from cleaning agents, air-borne contaminants, etc... Urethane primer, because of it's sealing capabilities for lack of a better term, is much easier to rid of the above mentioned crap. You can literally wash spilled oil off of urethane without any lingering effects... try that with polyester primer. Can't do it.
