Oh without a doubt, use an inspection report first and foremost!!
As for customer relations, smile! Gladly rectify the scratch at whatever makes the customer happy! Offer to refund the cost of the detail before they ask! If you value the customer, do whatever is necessary to keep them happy and coming back! Do not try to argue the problem, just offer sympathy, appologize and explain that you take every precaution to prevent ANY damage and quickly show how you protect the finishes.
I had a same problem, just offered " I'm terribly sorry, would you like the dealer or your bodyshop to repair this for you?" "I'll arrange for them to come to the site and give me the amount, then we can schedule for it to be repaired as soon as possible." "Again, I'm terribly sorry and I'd like to offer you (your choice on this one) a refund or another detail free of charge and one for 1/2 off for one of your other employee's cars or friends" "I want to insure your satisfaction with the results of my work" "Please understand that I take all precautions in preparing your vehicle and I neglected to do my pre-inspection with you prior to working on the car." " Let me call the dealer/bodyshop now so we can rectify this right away, and I'll contact you so we can get it in right away"
Now, the person was quite happy with my resolution and I gained so much work from this, as I stood behind my work, and offered to solve a problem that although was not in anyway caused by me, had it done quickly so the customer does not dwell on the problem long enough to be mad or upset. I scheduled and handled the whole thing, picking up the car and dropping it off.
I'm not in the business anymore as every Tom-**** and Harry started a detailing business in the area and just felt it was over saturation as some undercut prices etc.
Anyway, always walk the car with the customer, not so much to first point out scratches, but to see and show areas needing attention and how you will correct it. If you find a scratch, I used to use a china marker to highlight it before I took a pic of it. Sometimes I can get it out, other times it takes the shop to touch it up, or I'll do it at a later date.
Good luck, and keep the customer happy without loosing your shirt! You can tell a customer out to get something for nothing at times, and those you do not want their business!
Regards,
Deanski