Need help after cleaning and brightening

That is oxidization that was not removed fully. You cannot just spot fix. You would have to remove all and start over.

@administrator  What does remove all mean? Right now the surface is bare. You mean clean and brighten again and that should fix it?

If no stain on the wood, then you do not need to remove. Try going over the post by sanding or rewash.

@administrator I tried some sanding with different grits but most left it still showing. When you say rewash, is that clean and brighten or just water wash?

Try water only, but get a little closer to the wood when pressure washing. Basically, you are just trying to remove the oxidation "fuzzies".

@administrator Using the water didn't really change anything. Should I use a specific grit for sanding or should I just do the clean and brighten again?

Try sanding.

@administrator Sorry I wasn't that clear. I have sanded, but it didn't work. If I clean and brighten it again will that work?

Or was wondering if you had a grit you would recommend.

Not sure if you will be able to get it out on a fence. Normally when this happens we buff off the fuzzies but that is very hard on vertical wood. You could try the buffing by using a hand buffer: https://www.deckstainhelp.com/wood-fuzzies-furring-on-deck/