My son sent me this pic of his driver's side sill, showing some corrosion at a panel joint under the door.

I'd like to understand this seam better. Are the sills covered with a sort of vinyl coating? At one time I cleaned them up and sprayed with SEM trim black.
I had to zoom in on this pic to even find the seam, they're always too dark in most pics.

If you look at the panel construction on the sides, #4 "cover" is the sill piece and looks to go full length all the way to the rear wheel opening (?).
So what's the non-sill-shaped bit at the rear (right in drawing) end? Is the rear quarter panel installed over the top of that area?


And is the forward end of this piece the joint at the front fender? So where is this seam under the door? What's behind it?
Thanks -
I'd like to understand this seam better. Are the sills covered with a sort of vinyl coating? At one time I cleaned them up and sprayed with SEM trim black.
I had to zoom in on this pic to even find the seam, they're always too dark in most pics.
If you look at the panel construction on the sides, #4 "cover" is the sill piece and looks to go full length all the way to the rear wheel opening (?).
So what's the non-sill-shaped bit at the rear (right in drawing) end? Is the rear quarter panel installed over the top of that area?
And is the forward end of this piece the joint at the front fender? So where is this seam under the door? What's behind it?
Thanks -