I need to clarify my initial post here. I posted Never Dull as a great choice for cleaning/polishing metals like brass portholes, etc. for salt and fresh exposure.
If you are looking to coat and seal your brass, I do have a good option.
My current 2013 Ranger has brass that looks like new...... I wondered why it still looks so pristine.....well , the search mode on TugNuts answered my question. It seems that the original owner posted ( under Brass portholes) that he cleaned them with acetone....then taped them off...and sprayed with either Rustoleum or Krylon Clear . Well.....the brass looks like new.....BUT....mind you, this is FRESH water. I make no claims for salt. We all know what a mess shellac and varnish on teak is once it starts to break down. The removal and refinish rigor makes one wish they had opted for penetrating oil rather than surface film protection. I assume the same for surface sealants on metals in salt environments.
SO.....if you are fresh water folks , the Acetone/Rustoleum/Krylon Clear may be your answer for a no maintenance porthole.....Saltwater....?????????????????????????????????