How to properly refinish & maintain exterior siding behind circuit panel
I'm not sure what to do in terms of sanding down the surface, but, since nobody can see it anyway, sanding it nicely probably isn't an issue.
For the finish, I'd suggest spraying your finish (stain and/or sealer) behind the panel. You may need to mask up the area around it to prevent over spray onto areas that have already been finished.
If you haven't yet finished the rest, you could try pouring some of the stain/sealer down the wall, above the panel and just let it run behind. It certainly won't be 100% perfect, but since it's hidden from view and the main goal is protection not appearance, that shouldn't matter. Since the flood of liquid will make a bit of a mess beside &amp; below it, have brushes/rollers at the ready, and, as it's running by, make it look nice where it is visible. Now that you've got this wall wet with finish, do the rest of the staining on this wall before this area starts to dry and you end up with a line from where new stain overlaps old dried stain.