Strip and brighten to fix this. Use Restore A Deck prep kits. Stain with a water-based stain to prevent it from coming back. The Restore A Deck Stain ...
See here for the best solid stains: Pressure wash for prep.
You use the same products on a fence that you would use on a deck:
You should remove the current coating first.
Nice job!
Tung Oil will do nothng for UV protection and will attract mildew. See here about new wood:
Defy Hardwod is very good. Penofin and Super Deck are very poor. Do not last and both turn black from UV. Very poor.
Since a solid stain, you should be able to just recoat.
Strip and brighten all with the Restore A Deck Kits and both additives: You may have to lightly sand after but hard to say if needed or not until yo...
No issues doing it next Spring. We would wait.
It is either mildew to rust. Either way you would need to strip and start over to fix.
We would need pictures. Also, you should have treated this as new wood and you over sanded too smooth: