Getting a very nice job will be difficult without a professional grade spray gun -- spray cans are almost never equiped with adequate nozzles, and the mix of propellant and paint causes serious orangepeel problems in amatuer hands (witness the hood on the 280 if you could see it -- looks like sandpaper).

You will need to remove the paint and all the rust down to bare metal to properly repair the spot, including in the seam between the door panels if it got that far. Scrape all the loose paint (the blister) off and as much of the rust as possible. You can use coarse sandpaper, but don't get too carried away. If you have an actual hole in the metal, you will need to add some fiberglass behind it, inside the door or weld in some clean sheetmetal, paint won't fix a hole.

If you feel you need body filler to smooth it out, you MUST grind all the way down to clean metal, as Bondo and related stuff really only sticks properly to freshly preped metal. Don't wait overnight, sand it clean and wipe with laquer thinner, the immediately apply the bondo. Allow to harden, the sand smooth. Make sure to use very fine paper that removes all scratches or they will show in the finished job.

Prime, wet or slick sand, then apply several coats of finish. You will need to mask the rest of the car off with masking tape and newspaper to keep overspray off.

You must cover any exposed bondo on the back of the panel with paint. Perferable not to have any showing, of course, but it is porous and will absorb water if not painted, and in a couple years, rust will appear under the bondo and force it off.

If you choose not to use Bondo or other filler, use an etch or conversion primer first, then without sanding, prime, sand, and paint. Make sure you don't sand down into the conversion primer, you will probably get rust recurring.

Peter

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1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!