Take this with a gram of salt. My 07 Jeep GC has a plastic bumper topper that is pretty grey. I wanted to coat it, but not until I used trim restore on it. I put multiple thin coats on and it turned out great. To me coating it after the restore dried for a day or two seemed too soon. The restore looked too wet, so two weeks and two car washes later I coated the plastic.
My point is, this may take multiple visits to get it back. I would let the customer know you will make it right, but it will take multiple visits.