Pour some turpentine on an open cut, and you'll know that stinging comes from more than acid. Simply put, you always want to prime the pipe that will be under 24/7 water pressure. The cost of joint failure always exceeds the effort to get it right the first time. The only people that might get away with not priming are those warm-climate folks that use pipe that is only weeks old, and not at all dirty or glazed. Sandpaper is not a substitute for primer.
