As soon as brass is exposed to the weather it will tarnish. To keep the polished brass look its needs to be regularly polished from the start using a standard brass cleaner and following the manufactures instructions.

To prolong periods between polishing, the exposed brass may be coated lightly in Vaseline or Olive Oil . This will not detract from the appearance of the plate yet reduces the rate of tarnishing.

An alternative is to for us to apply a lacquer. Although this does delay tarnishing but will never completely stop it and the lacquer itself, also deteriorates over time.
Future brass cleaning would require removal of the lacquer with a paint stripper. This is not a problem with the unpainted engraved plaques, but would remove the paint infill on signs with coloured letters.

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For maintenence free signs you might consider the gold coloured engraved exterior laminate or anodised aluminium signs