I am restoring a 1962 Impala. I’ve taken the floor pans out and found rust where the rockers meet the inner metal below the dash. It is rusted completely through. Once the new floor pans are back in, and a couple of repair panels are welded in on the rockers, you won’t be able to see this area. I can’t get to it to wire brush it or otherwise clean it. They don’t reproduce this particular piece. I’ve read about all the products. What is my best option to be able to spray back in these areas and stop and seal the rust. It’s very jagged and rusted. Thanks in advance for any help.