Did you try hammering them? Lump hammer placed on one side of the rivet, then hit the opposite side of the rivet with another hammer. It works more often than not, but if not try Loctite Marine Adhesive and sealant. Wire brush or sand the area until shiny, and wipe with rubbing alcohol and allow to dry before applying.
PS, once you open the tube of Loctite Marine it begins to harden, within 24 hours it's unusable. Squeeze the tube in the store before buying, to make sure it's still soft inside! HD carries it.