In my opinion, that is coming through from the inner side.
And you know what THAT means!
My guess is that the leading edge of the sill and the lower edge of the guard are requiring close inspection and repair
fairlady 088.jpg
Probably not this bad but you get my drift
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Agree with the rust converter. I like Sems Rust Seal... Used it throughout my very rusted car 30yrs ago and the places I can see are STILL rust free (black coating). Do investigate some about what may be happening behind the fender. Dirt and crud build up ther and hold moisture and slowly rust out the other side. So, at least clean it well, blast some compressed air in if you have a compressor. Then, if you want, you can use a spray can based rust converter or use compressed air with the liquid converter and "air force" or blow the converter into the little crevices on the back side of the fender where it connects to the body. It doesn't have to be pretty, the goal is to get the converter into small spaces without having to remove the fender or risk breaking off a rusted mounting bolt to gain space. Just a thought, but that method worked well when I needed to push converter into places I could not easily reach. just be sure to select a converter that does not require that it be top coated or painted afterwards (that's why I chose the Sems Rust Seal and not Rust Mort).