How do you cut such delicate small holes like in that last repair?
Ha this could be a trade secret??? errr no i used one of these
right angle high speed grinder with a oval shaped grinding burr. I drilled a couple of 1/4" holes one each end of the area to cut out then using this wee beastie opened the holes up until i had ground out all of the rust. We use these a lot in the Aviation industry especially for corrosion removal.