One way to avoid rust is to replace the metal bulkhead with a fiberglass, plastic or wood one. Barring this expensive alternative, there are ways to get rid of or at least minimize the rust.
Remove the Rust
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Scrape away any flaking rust and paint with a paint scraper.
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Sand the remaining rust with heavy grit (80 or coarser) sandpaper. If you are unable to remove the rust by hand, use an orbital sander or grinder to get the rust off all the way down to clean metal.
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Replace any bolts, screws or other hardware that have become rusty. If you leave these, they will cause the rest of the metal to become rusty again more quickly.
Repaint
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Repaint the bulkhead with a high-quality, exterior, oil-based primer. Pay particular attention to edges and hard-to-reach places, since these will tend to be the places that begin to rust.
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Paint over the primer with an oil-based exterior metal paint.
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Give the bulkhead several coats of the finish paint for maximum protection and to delay future rust formation.