if it's your dd, keep the car outdoors after you drive. let it cool down and freeze.
dry salt doesnt do much harm. but melted snow/ice (from garages/heated garages) combined with salt =
i speak from experience.
best way to keep your Z clean in the winter is hibernation.
as for additional undercoating, dont bother. just go under the car every spring and wash the undercarriage very well, and wire brush the surface rust off, clean, and just spray some rustolium in the affected areas.