Thanks for the suggestions. I am planning to coat the area with POR-15 and use a 3M seam sealer on the joint when I am finished. What about trying to wick some thinned paint into the seam after welding? I would like to do any thing I can to protect this seam, and to treat the seam on the opposite side of the car. Even the rust free parts car I cut the donor panels out of had a bit of rust beginning to form in this area.

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1979 240D- 316K miles - VGT Turbo, Intercooler, Stick Shift, Many Other Mods - Daily Driver

1982 300SD - 232K miles - Wife's Daily Driver

1986 560SL - Wife's red speed machine