Plus ten on Clay and wax. You definitely want wax after the clay, and hopefully a sealer atop the wax (they make combo products, but I'm old school in that I like them separate).

The clay is really a cleanser. It lifts any old wax as well as surfactants, residue and debris, leaving you with a thoroughly clean surface. If the paint is in otherwise good condition, it's well worth the cost to clay it before waxing.

Good luck.