A car is an endless drip of maintenance, fuel, and depreciation. Daily driving ruins clear coats fast with road salt, sun fade, and swirl marks from sloppy washes. A proper ceramic coating stops that wear, but shop labor is not cheap. Detailers usually see business dry up in late winter, which makes January and February the best time to ask for a deal on bay time.
Negotiate the job on its own merits before you think about payment. Once you lock in a fair price, you can treat the upgrade as a fixed monthly project. Using the Citi Simplicity® Credit Card lets you handle the out-the-door shop cost without burning cash up front or taking on interest.
That 18-month window is an absolute deadline, not free money. If you still owe anything when the promotional period runs out, regular variable interest hits whatever balance remains.