NXT is not a good last step product. It will clean off whatever you have put on prior. It wouldn't hide swirls for 3 months anyway. No product would. I wouldn't hide swirls though. I'd point them out and let them know how much it costs to get rid of all of them. Honestly, and no offense intended, but hiding swirls is for amatures. If you know how to remove them then you should do it. If the customer isn't willing to pay for the time it takes to remove swirls, then you should leave them the way they are so that they finally come back to you and ask to have them removed. If I took my car to have it detailed and it came back looking great but gradually went downhill from there because all the defects were merely hidden and not removed then I would be pretty mad. If you can get rid of 95% of the swirls then I wouldn't worry about that last bit. Most people don't even see swirls anyway. Detailers do, but the average Joe has no idea that their paint can look better than it does.