
Bloody hell its just like a Maestro!
This seam is not a million miles away from the C pillar join on my Maestro. From factory it should have a thin smear of seam sealer, on my Maestro, all external seams like this had nothing to stop the water getting in.
Because its spot welded the seam will not be perfectly flat, and a build up of moisture and dirt will form, especially if you are not keen on washing the door shut out.
Here a bodged Maestro one, it had been plastered with filler over the years making it worse. The Maestro in question was well known and i had to do lots of welding for the chap to make it good.


My advice would be to keep it clean, and apply a coat of waxoyl over the seam if you want a quick and easy way of holdling it back. Next step would be to clean all the paint off the area and use a rust killer, zinc/anti rust primer and a coat of seam sealer, then your usual paint finishing. Inside treat the box section with Waxoyl or similar.