Both, in different ways. A pre-winter detail (October/November) seals the paint and undercarriage with wax or Jet Seal protection — that's what stands between road salt and your clear coat for the next 3 months. A post-winter detail (March/April) removes the salt residue and any micro-pitting before it gets worse.
If you can only do one, choose pre-winter. Salt damage compounds — every winter it locks in deeper if not addressed. The pre-winter wax or Jet Seal refresh is preventive, which is always cheaper than corrective work.
Customers who park in heated garages year-round can skip the pre-winter detail; customers who park outdoors need it.
