I discovered that the upper tailgate from my MY2010 Westminster is slightly corroded, what a surprise.
There is only corrosion on both edges on the inside (seam?) and around the locking mechanism.

I already used a can of Dinitrol ML to treat the inside of the body panel.

How should I proceed to treat the corrosion in the area as shown in the picture?

Click image to enlarge

hello, i know this is an old post but id love to know of you managed to correct this and if so how?

Once the rust is visible on the inside of the tailgate then it is too late to treat it successfully, anything you do will just "slightly" postpone the inevitable, the only way to fully cure it is to cut off the bottom outside section of the tailgate, sandblast all the rust off the frame and weld in new metal then treat it all to a good dose of rust prevention...

The rust has developed between the outer and inner skin and once it's there you can't get rid of all of it without surgery... it only takes one crumb of old rust left behind for it to continue to rust....

Keep an eye out for an early tailgate with no rust, the early ones were made out of better quality steel and had some rustproofing....  Pete
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2014 L405 Autobiography SDV8 4.4 Loire Blue Ebony interior
2011 L322 Vogue SE 4.4 TDV8 Baltic Blue. Parchment over Navy Interior. Sold
2012 L322 Autobiography 5.0 Supercharged Ipanema Sand, Jet Interior. Sold
2002 L322 Vogue 4.4 V8 Epson Green, Ivory over Aspen Interior (Fatty Offroader) Sold
-Click for Project Fatty off roader-

A rusty tailgate is a badge of honour. It proves you are a dedicated L322 enthusiast and not just any old SUV owner. 2012 TDV8 Vogue (L322)

That did make me chuckle and l totally agree!

Depending on how bad it was it could have maybe been 'saved'. Depending on how the rust was on the inside of the tailgate. I'd say your best bet is to have a shop grind all the seam sealer off and open the lip to see how the rust is in there seam. Hopefully if not bad or just concentrated around the seam then you can treat it, reapply seal sealer, and repaint and get some more time. But I HIGHLY suggest a yearly treatment of cavity wax of sorts in the upper tailgate.

Otherwise your only option is to find a new/used tailgate that is rust free and start treating it from there.