body cavity wax spray applied generously, or route the condensate drain a bit to the rear of the car. If the battery overflow is causing acid to drip on the front cross member, you need to make sure the original plastic tray for the battery is present, it also would have a nipple fitting to attach a pipe on it, that pipe should hang beside the cross member so all acid drips down.
btw - if you do get front rusting, check why is your battery overflowing.