I'm doing a quick repair to the mini deluxe as it has started to develop nasty rust along the gutter.
I have used a small rotary wire brush on drill and also dremil but the PO had also put body filler and seam sealer in the gutter and even though I have managed to get most out of the gutter it is now exposing the rust between the roof and body panels.
I think the ultimate would be to remove the roof and clean both surfaces but that is way too much work and probably open up a can of worms?
Sand blasting the shell would also probably prepare the seam for repainting properly?
Any suggestions?
I like to clean all of the rust away as if not done properly the rust always comes back.

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1969 Cooper "S"
1967 Mini Deluxe
1973 Clubman Van (the fleet spare)
1978 ex 1275 LS ("Wizard" Eaton Supercharged)
2015 HSV GenF GTS(occasional drive & tow car)
2019 MINI F55 Cooper S

The rust will most likely be between the lip and the gutter, but in the lip itself. I just removed mine from the Crudman, as one side was bad. I felt underneath to find the spot welds and ground them back. It looks better without it, and in my case I'm not interested in the originality.

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Hi Poeee,
Not much rust between the lip and the gutter so I am not planning to remove the lip/drip rail.
Rust is between the roof and the gutter.

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1969 Cooper "S"
1967 Mini Deluxe
1973 Clubman Van (the fleet spare)
1978 ex 1275 LS ("Wizard" Eaton Supercharged)
2015 HSV GenF GTS(occasional drive & tow car)
2019 MINI F55 Cooper S

Roof flange was continuous resistance welded (overlapping spots) to the side panels using driven copper rollers on a BIG fixed spot welder.
Good luck getting the panel off, most Minis with BAD roof damage got cut & shut through the A, B & C pillars. Neatest way to do it when bent.

In your case, I would use a dremel to cut the rust back inside the gutter to clean metal, then braze it.

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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.