I would take the cover strip off first.
Cut the outer panel hole bigger than needed to get beyond the rust in the inner panel.
Weld the inner panel patch in.
Paint the inner panel as much as possible with coldgal paint or zinc-it primer.
Cut a patch for the outer panel and weld it in. Paint the inside of it first with coldgal or primer but keep back a bit from the edge.
I'd get all the crud off the inner panel, including up and around the door hinge brackets, paint it all in there with coldgal.
Next best thing to hot dip galvanising.

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DrMini- 1970 wasaMatic 1360, Mk1S crank, 86.6HP (ATW) =~125 @ crank, 45 Dellorto (38 chokes), RE282 sprint cam, 1.5 rockers, 11.0:1 C/R.
