How to Remove Rust from a Car
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There are many reasons for a car to develop rust spots. The main reason is when the paint is chipped or scratched through the primer coat, perhaps by stones on the road or a minor car accident, and the metal underneath is exposed to moisture and air, which causes the metal to oxidize. A rust spot increases and spreads with time, especially with each rain or snow storm. Whether you plan on keeping it or selling it, your car will look cleaner without the rust. Others judge you by your own appearance, by your speech, and by the car you drive, so take the time to remove the rust spots and give the car a painting before the entire car ends up covered in rust. 
- Take safety precautions. Wear gloves, safety glasses and a dust mask.(There may be some extreme situations that you may need a respirator.)There will be fine rust and paint dust as well as paint spray everywhere; you don't want it in your eyes or l 1_Cover up any parts of the car that you don't want to get dusty. Use a tarp sealed with painter's tape to define your work area. 2_Masking the car off is a very important step that can go wrong in a lot of ways. Do not use newspaper as shown in the pictures; paint spray can leak through it and leave specks on your clear windows. Use something less thin porous like real masking paper. Also, simply covering up a surface is not enough; paint spray can find its way underneath edges, so you must tape every single edge of your masking paper down, not just the ones needed to get it to stay in place. 3_Do not actually place your masking job like the way that is shown in the pictures; you don't want the masking to stop in the middle of a panel or you will be left with sharp lines like those shown in another picture here. These lines do not go away with any amount of buffing or addition of clearcoat layers; they will be permanent if you do it this incorrect way. The only option is prevention by masking the car correctly in the first place, by stopping at panel lines and going no farther inward (or better yet, stopping a few panels away so there isn't a drastic color difference between one panel in the next, but this requires learning how to gradually blend paint, which is done when spraying, not after the fact) 4_Remove the paint around the rust with DA (dual action) sander. A DA sander gives you control over the speed of the sander while removing the paint. Start with 80 grit and work you 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. Feel with your fingers for a smooth surface 5_Switch to a metal grinding wheel for removing the thick rust and getting into any pits. When using the wheel, go slowly, because these can do a lot of damage. Once this is done apply rust removing acid to the area to remove the microscopic particles of rust that remain. Phosphoric acid is 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 off by sanding by hand (120 grit sandpaper) to get a nice smooth metal surface. 6_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.
7_Prepare the spot for priming. Follow the instructions for your primer. Typically, what you will need to do is: Wipe the area with mineral spirits or paint thinner.
8_Spray three coats of primer, waiting a few minutes between coats. Allow to dry over night (at least 12 hours) 9_Sand with 400 grit wet sandpaper, This abrasive is specifically made for sanding between paint coats to smooth the surface and degloss, so the paint bonds. Keep a bucket of water handy to rinse the sandpaper frequently, so it does not get fouled with paint. Wash painted area with light soap water mix. 
10_Spray a thin coat of paint 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 it set at least 24 hours before pulling off the tape.The Rust Repair Process Explained...
- If the rust spots are on or around the fender, it may be useful to jack the car safely with a chock behind one of the wheels. Pull off the wheel and unscrew the plastic protecting the wheel well. Doing so will give you a chance to pound out any dents from the inside, and will also allow more room for grinding and painting.
- Propellants have explosive properties, so do not allow any sparks or flames, including lit cigarettes, near the work area during the entire rust removal process.
- Instructables - Article by User:Intoon. Source of information and images. Shared with permission.
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