An autobody shop told me once to use this window primer sealer that usually sits in the window frame prior to placing the glass...I tried it on some spot on my SW buymper that always had snow build-up in and slowly wore away the paint to start surface oxidation...so far it's been great but once it's on anything, it's not coming off...it's very thin as far as application but it has done it's job...not cheap but worth having in the garage for other issues with rust:
Or you can POR15 it and repaint the correct color...or look for another tailgate from a wreck...not a fun venture but not uncommon...