Fixing a rust spot on the Jaguar XJ6

In front of the driver's side door, moisture has gotten behind the back of the fender panel and caused the metal to rust from the inside out (i.e. from the back, unprotected side to under the paint).  As the rust forms, the paint "bubbles up":

 

Before grinding to remove all of the existing rust, it's wise to mask off areas that need to be protected:

 

With a wire "cup brush" on an angle grinder, the rust can be easily "whisked" away and the paint surrounding the area removed.  Once the rust was removed, a good size hole is visible:

 

In stead of cutting a little piece of sheet metal and welding that in, I patiently "stitched" the hole together with my MIG welder.  This takes lots of stop-and-go welding to allow the heat to dissipate so you don't "blow through" the sheet metal.  In a situation like this, where the metal is thin due to rust, you will blow through it some, but eventually, as you work away from the rust hole, you'll find metal thick enough to support the heat of the weld.  Here's what it looks like once the welding is done:

 

Now comes some patient grinding to smooth that big welding "blob" down:

 

Once the grinding is done, you can inspect your work and see if any other pin holes need more welding attention.  On this specific patch, it took three "weld-grind-repeat" cycles to get it right:

 

And finally, a coat of rust preventative paint (in this case POR-15) is put over top the bare metal (and behind the panel, too):

 

The next step, which will come later, will be to put a skim of body filler over our patch then carefully sand it down so it matches the contour of the car exactly (or as close as possible).  Then it can be painted with the paint blended in to the area around.