1.   An option in this case is to paint it with a zinc-rich primer and finish with an oil enamel.

2.   I plan to paint them, sanding and washing first, then using an oil primer and finally an oil enamel.

3.   Third, sand the rusty cover to remove all rust, then paint with an oil-based primer and finish with an oil enamel.

4.   While fiberglass generally requires a specific paint, you could try painting the stain with an indoor-outdoor oil enamel and hoping for a disguise, at least.