Wow - what an amazing turn around. It pains me to see the scratches, especially in a black, but you took the challenge, making it all better.
Beautiful transformation and further proof that I need to add more money to my Rupes "money jar"...got to get one of those.
A very nice turn around. Sorry about the less than excited response from the owner - sadly, I've gotten that same response from some when returning a car after fixing neglect. I always take away that I did my best and know that somewhere in there the car is jumping up for joy...can't hear it...
Here is a short list of what may be causing it. It sounds like you have determined inside faucets are fine but, outside is an issue.
Check the outside faucet for a back-check device. It will be brass and threaded on the end of the hose bib where you connect the hose end. I've seen trash build...