Tarnishing is caused by contact with the air that we breathe, or more specially oxygen. Although I am not sure of a simple way to remove it after it has occurred, I do know how to prevent it. If you put the assemblies into sealed bags and fill the bags with nitrogen prior to sealing that should stop the tarnish from forming in the first place. Use a standard ESD safe bag that can be heat sealed.

There are off the shelf inexpensive systems for doing this, contact your local supplies distributor. When you are ready to assemble the board simply remove the bag.

You may have to use a higher solids flux that is more active to remove the oxides. This could also require that you clean the boards after wavesoldering.

Try increasing solder pot temperature and going to a more aggressive flux formulation (maybe moving from a no-clean to an aqueous). This might be an easier approach than attempting some type of rework process.