FAA Requirements for Aircraft Exterior Cleaning

Washing, waxing & polishing are preventive maintenance items listed under FAR part 43, appendix A, section C, paragraphs 9 & 10, and also under FAR 1.1 (preventive maintenance).

You'll find the "Aircraft Cleaning" listed as Preventative Maintenance in the A&P handbook which outlines what Preventive Maintenance is: A&P handbook Ch. 6-19 thru 6-25. Both interior cleaning and exterior cleaning are listed!

The FAA requires that all preventive maintenance items for FAR 91, 121 and 135 operators need to be completed and singed off by a certified A&P mechanic for approval to return to service in compliance of FAR part 43.3.
You will also find aircraft exterior cleaning as a maintenance procedure listed in your aircraft manufactures maintenance manuals. Again, any procedures listed in your maintenance manual needs to be accomplished by an A&P mechanic along with a logbook signature.

The only difference when cleaning part 91 vs. 135/121 aircraft is the pilot/owner can perform the work (only on their own a/c) but the FAA does not allow for anyone else to perform the work unless they hold an A&P / AMT certificate.

Don't let the local FAA down your aircraft because you used an auto detailer! Use professional A&P mechanics... It's a federal law!