I would wash it with dish washing soap.

Clay the car. Clay will remove contamination that is embedded in the surface. It does something that paint cleaners and polishes cannot do.

Put down a Polish

Put down a Wax/Sealant like Meguiars NXT or a straight sealant like Wolfgang or Klasse.

Take your time and do all these steps and I think you will be pleased with the results after a snow attack