Question About Sanding

Hello All,

I am sanding the 68 cuda (keep in mind that I have no clue what I am doing. So I am taking everything one step at a time. So I am working on the underside of the trunk lid. There is a lot of filler or sealer along the edge seal where the front metal overlaps the back metal. I cleaned it off with a dremmel. I am using a DA sander for most of it and the dremmel for the areas that the sander doesn't go. I am using 80 grit sandpaper for the initial removal of paint and bondo. And switch between the 80 and 220 grit. It's all I have now. There are tons of little pits and holes and dents. The dremmel helps remove the filler inside these dents. The metal along the edge is all pitted but not eaten through with rust. I can't seem to get out every little bit. If I keep sanding, there will be nothing left. This is all on the back side of the lid. How do I know if I got ALL the crap removed and what happens if there is a little bit along the seam where a tool can't go? How important is it to get everything perfectly smooth on the backside under the trunk lid? I can understand the panels have to be perfect. I have been using these tools and hand sanding to the point that my hand is constantly cramped. My husband said it doesn't need to be perfect because no one is going to see it but I need the practice anyhow. We can't afford a sandblaster right now so I need to do this all by hand. Any tips about sanding? Ideas, helpful hints? I have been viewing videos on youtube about body work and sanding but need to know what you all do.

Also, along the curved edge of the inside metal along the lower right corner, it has rotted through causing holes. How do I know if I should weld it with solder or replace the metal. It is going to be pretty hard to get that shape from a straight piece of sheet metal. Any suggestions?

Any help would be appreciated.
Joanne