The picture is looking at the driver's side rocker panel - a rust-through spot.

Compared to my previous 21 year old BMW, this 240 is in great shape rust-wise. I am showing the second rust spot on the car (the first is where broken antena mast was pushing into the rear wheel fender). This one was there when I bought it - I noticed the filler job right next to the rusted hole.

The question is, why did it rust in that spot? All other rocker panels are in great shape, knocking sound solid.

For now I stuck some silicone into the hole and kept a strip of tape over it. I think to fix it I am going to carefully grind around to clean metal and then build up a skin with carbon or fiberglass and epoxy (I am more used to boat repairs). Should I be overly concerned about this spot and perhaps fix it with metal plate welded in its place? Should I inject rust-cleaning spray through this hole when fixing it all up?

Thanks as always!