A quick soak in EvapoRust will brighten them up. Maybe 10-15 minutes.(you can experiment with different times) The zinc is looking tarnished because it's doing the job it was meant to do. The zinc is a sacrificial coating. It's being eaten away instead of the fastener. I'm not sure how long the brighter appearance will last once you remove it from the EvapoRust. It just depends on how much zinc is already worn away and how much is remaining on the fastener.