another question - inside the door on the back of the door skin there is a non-smooth surface(maybe undercoating??) but not sure if I should try to remove or leave? is it normal?

does anyone know if it would be sprayed from the factory like this? it doesn't look like rust more dark rough surface.

 

I wouldn't be surprised to find some kind of coating in there. I don't know much about it, but I'd say its common. Well, it was common in the old days to have these cars treated, sometimes at the dealer or at a different shop. Names like zeibart and ming come to mind.