I replaced the quarter panel on my '69 due to a Mazda-shaped dent in it. It is a 3/4 full repro quarter that does not go all the way to the factory seams. I chose to do a lap weld with about 1/2" overlap on the seam spanning the length of the quarter.

I am wondering what to use to seal the weld from inside the trunk. My dad, who owns a body shop, thinks I should use seam-sealer applied with my finger and try to push as much between the panels as possible.

I am worried that this might leave pockets to trap moisture in and rust the weld from the inside.

My idea is to use rustproofer and try to spray it into the gap between the panels, but we aren't sure whether this would be the best way or not.

It is not in a location where road grime is a concern, but in my area humidity can be a serious problem.

Derek--Have the same problem on my Elky (#1), bad part it is the reverse of yours.
Lower 1/4 area, panel was glued from w/o to lamp, gap will act as a ledge to catch material.
Am planning a similar approach to yours. Will use a wax based material, made by Wurth, it is thick. Will spray in under pressure, totally fill void with product.
With the type of use that #1 will be subjected to, I am confident it will suffice.
On yours, my sense is that you could use your idea, or your dads' and be just fine.

Karl