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This method involves using a sander and a grinder — two powerful electric tools that can kick fine rust and paint dust into the air. To avoid injury and protect yourself from these airborne particles, be sure to wear gloves, safety glasses and especially a dust mask to keep the rust and paint particles out of your lungs. For heavy-duty jobs, consider using a respirator rather than a simple dust mask. As noted above, this job puts rust and paint particles in the air. If you're not careful, these can settle on your car, giving it a "dirty" appearance that can be hard to get off. To avoid this, "mask" the parts of your car that you're not working on (that is, cover them with tape and masking paper.) Use a tarp sealed with painter's tape under the car to define your work area and protect the floor. Masking the car off is a delicate art.  Do not use newspaper, as paint spray can leak through it and leave unsightly specks. Instead, use real masking paper, which is less porous and won't let paint through. Also, be sure to tape every single edge of your masking paper down. Don't just use a few small pieces of tape to get it to stay in place — paint can (and will) work its way under any loose edges. In general, you don't want your masking to stop in the middle of a panel, or you will be left with sharp lines where your new paint ends and the old paint begins.  These lines do not go away with any amount of buffing or addition of clear coat layers, so practice prevention by masking the car correctly in the first place, stopping at the panel lines around your rust spots and going no farther inward. If you're experienced with auto painting, you might alternatively try stopping your masking a few panels back from your rust spot. If you know how to gradually blend paint, which is done when spraying, you can use this tactic to make it so that there isn't a drastic color difference between one panel and the next. A DA sander gives you control over the speed of the sander while removing the paint. Start with 80 grit and work your way up to 150 grit. Use the DA sander with (80-150 grit) to take off both the primer and paint, as well as any light rust that hasn't fused with the metal, and level the surface between the painted surface and the unpainted area. After you're done, feel with your (gloved) fingers — you should now have a smooth surface. Next, use a metal grinder to remove any thick rust buildups and expose any pits. When using the wheel, go slowly, because these tools can do a lot of damage to the car's body if used incorrectly. Once the grinding is done, apply rust removing acid to the area to remove the microscopic particles of rust that remain.  For this job, phosphoric acid is usually best and can be bought at most auto parts stores. If you want, use a hole spot filler or a body filler like Bondo to even out some of the dents and fill the space where the paint is gone. Finish application of your filler by sanding by hand (using 120 grit sandpaper) to get a nice smooth metal surface. See below for more extensive information on using fillers. Purchase primer that is ideal for painting on bare metal and an auto spray that matches the color of your car. Both of these supplies can be found at an auto supply store. Primers may vary, so follow the instructions provided with your primer or talk to an expert at the auto store for definitive information. Typically, what you will need to do to prepare is:  Wipe the area with mineral spirits or paint thinner. Tape newspaper on all surrounding areas within three feet. Spray three coats of primer, waiting a few minutes between coats to allow each one to apply. Don't over-apply — there shouldn't be so much primer in any one coat that it drips or runs. For most primers, you'll need to allow the fresh coat to dry overnight (at least 12 hours). This abrasive is specifically made for sanding between paint coats to smooth the surface and degloss so the paint bonds properly. Keep a bucket of water handy to rinse the sandpaper frequently so that it does not get fouled with paint. To finish, wash the painted area with a light soap and water mix. Use thin coats of paint and let each coat "rest" for a minute or two between applications so as not to let the paint run or sag. Use as many coats of paint over the primer as you need in order to achieve a nice color and finish. Let the paint set at least 24 hours before pulling off the tape. Be patient — if the paint still feels tacky, you may need more time. If necessary, apply a clear coat to match of the finish on the rest of the car. Finally, allow the paint to cure for 48 hours. Congratulations! Your car is now rust-free and ready to ride. As a precaution, never wax fresh paint within 30 days of painting — the scrubbing, buffing action can pull fresh paint off.
Take basic safety precautions. Mask any spots that you don't want to get dusty. Try to mask along the existing panel lines. Remove the paint around the rust with a dual action (DA) sander. Switch to a metal grinding wheel. Prepare the spot for priming. Apply thin, even coats of primer. Sand with 400 grit wet sandpaper. Spray a thin coat of paint. Buff the edges of the new paint so that it blends with the old paint. Wash and polish the car.