What I like to do and it's probably more of good luck thing, like in Hockey where player's don't shave in the playoffs, football they bump helmets after a good play, high fiving the wife for being tolerant of your bad habits and table manners...just before I apply the primer, I give it a going over...take another look at it to make sure that all spots have been sanded....and it opens the E coat up again...I'm sure it's just superstition but...it's never a bad idea to go over your work...several times I've found that small area that I had missed before.
The purpose of sanding the bumper with detergent is to get the mold release off and to gently rough up the plastic for the Epoxy primer. Concentrated more on the outside, we won't be applying much paint to the inside, it's rubber/plastic and won't rust. The Scotch Brite will seem to not dig in on the plastic as well as it did on the E coat, that's Okay, just make sure that all areas are scuffed.
Yes, the respect you are gaining for the body shop guy...and I've always been amazed at how in this trade, the most important job, prepping and sanding, is usually given to the guy with the least amount of experience. Good preppers are worth their weight in Gold...and real hard to find...I always tried to show the preppers I had the utmost of respect...a good prepper can make a bad painter look good, but, a good painter can't make a bad prepper look good.