When should a ceramic coating be reapplied, and can new coating go over the old one?

Reapply when the coating no longer recovers after a proper decontamination: water behavior stays weak, the surface feels grabby and dirt clings more than it used to. If the old coating is still partly intact and the paint is in good condition, some coatings can be refreshed by cleaning thoroughly and applying a compatible layer on top. If the paint needs correction, or the old coating is patchy, polishing first gives a cleaner base. Follow the coating manufacturer's guidance on recoating.