How Do You All Paint Wood When the Nail Heads Are Rusting?

Was going to paint a leftover wooden mold/form that was previously used to form a cement arch (this will now be a "bridge" for dog agility course). However, it was assembled with nails and now they have begun to rust. Painting over the nail heads must be a no-no, but I don't want this to become a major project. Anything simple and effective out there? Thank you.

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