Reappearing Deck Screw Rust Stains

Hello,

I am hoping that you can help me. I read another post on this forum from a few years ago in which a forum member was having the same issue that I am:

I used a wood brightener containing Oxalic Acid to clean my fully covered cedar decking on my screen porch and mostly to remove the rust stains that are on it because the builder used non-stainless steel screws. In my case, I actually REMOVED the old screws before cleaning because I'm going to replace them with stainless. When the wood was wet, and through the drying process, it was clear that the chemical HAD worked - perfectly in fact. HOWEVER, after the boards had dried, the stains came back.... and I'd say they were even larger (albeit faded). I repeated the process, again it looked like everything had worked, but again the black stains are back. I've attempted to attach pictures for you to see an example.

Anybody know what I can do? I'm not planning on staining it... and it has essentially zero exposure to moisture.

Thanks