W124: rust at the edge of trunk door

My 1992 Mercedes W124 300D 2.5 turbo sedan
has rust at the edge of the trunk door (blow the rear plate).

A technician said Mercedes is known for this problem. By design, the edge of the trunk door collects moisture. So rust develops.

He said that the repair cost of the edge could be quite high because of the following reason.

Two metal sheets meet at the edge. We can directly see rust only at the outside edge, but we also have too see how extensive the rust is, that is, whether there is rust in the inner side of the two metal sheets. So one has to disintegrate the two metal sheets, repair rust, and re-assemble the two metal sheets.


Hhm.
I thought of applying POR-15 at the edge of the trunk door myself. However, if there is rust on the inner side of the two metal sheets, rust will grow there.
I do not think I can disintegrate and re-assemble the two metal sheets my self.

Shall I have a body shop disintegrate, repair rust and reassemble the two metal sheet?

Or shall I merely apply POR-15 at the outside edge now. If the rust grows inside later, and make a large puncture on the trunk door, then I may have to exchange the whole trunk door. That will cost about $1000 including labour cost.

Which is better?

Or do you have any other suggestion on how to fix this inexpensively?