Id rub that with acid and steel wool.If it didn't come out of the pits with that,Id tape paper towels on there,and soak it with acid.Or see if there was a comparable jell type product.Or lightly sandblast.
If all the discoloration came out,Id seal with epoxy,then fill over it,as long as none of those are HOLES,or thin enough to almost be.
If you cut and patch,youre going to have to find,or cut a hole to provide access to the back of the weld area to seal it anyway.
I would see what the outside holds first,before cutting.
you dont have to replace that rocker thats in great shape, just treat it with acid or vineager with a scrubby pad and you will be just fine. make sure to give it a couple of coats of epoxy primer afterwards. theres lots of great vids of this on you tube from the pro's.
Do keep in mind that some epoxy primer makers strongly advise against spraying their product over metal that's been treated chemically. So read the spec sheet of whatever epoxy primer you intend on using and make sure you do what's right.
I still stand by media blasting as that removes all the rust and scale.