[TboneS54]

You do marr the paint with clay. Every time you touch the car, you marr it, even if you can't see it.

However, that would only really matter on a perfect finish or recently polished. On a typical car that hasn't been polished yet, claying it won't impart any noticealbe marring over what is already there.

If it were recently polished, and you didn't want to pay to redo it, but wanted a super clean surface for new wax/sealant/ceramic, I would advise against clay, and suggest only a chemical deconn aka iron/fall out remover spray.