The primer doesn't matter you can use EPX 900 or whatever under it and will be fine. You can spray Concept over omni or omni over concept. Just make sure to wet sand the piece back down correctly or scruff it to hold paint again. IMO, PPG needs to be sanded down with at least 600 grit to not show sand scratches. The paint rep here said he does it with 320 or 400 wet but I can't see nor believe that since I still see the sand scratches from it. don't forget, there is also Omni Plus now too.