Auto Body Repair and Repainting

Description

This page gives a rundown on how to do body work on pretty much any car. It includes procedures on how to repair rust damage, holes created by rust, dents, and re-painting panels that are already painted. These methods are for the do-it-yourself type and require only commonly available materials. Professional body shops use materials and methods the can do a better job (such as chemicals for bare-metal preparation), but for those of you who would rather tackle it yourselves or cannot afford to have it done, this page will guide you through the process. This is not a complete procedure and many people have they're own methods. I do recommend purchasing or checking out a book on the subject to get more details and suggestions, but this is the procedure I usually use. Please read through it completely and decide what materials you are going to use before you even get started. What steps and materials you'll need to use will vary depending on what you are going to do. When sanding, always use a rubber sanding block unless the area is small (then just hold the paper against your finger).

Preparing The Surface

This step prepares the surface so that repairs can begin. The amount of preparation that is necessary, depends on what type of repairs you are doing. If you are just going to paint or repaint a replacement body panel (because it is unpainted or the wrong color), use the Repainting A Panel method below. If you are repairing rust on a panel, use the Repairing Rust method. Proceed to the Patching Holes section if you had to remove sections of badly rusted metal.

Repainting A Panel

For a panel that you are simply repainting, you can use these steps. If there is some minor rust, see the Repairing Rust section below to find out how to clean it up before you repaint. These methods are typically used when replacing a damaged body panel with another (from a used parts dealer or junkyard) that is not the correct color. Repainting an entire panel (such as a fender or hood) requires an air compressor and spray gun to be done right. If you don't have access to such equipment, then it would probably be best to have the panel painted by a body shop. Color matching is always a tricky business, though.
If you purchased a new panel that has been primed, follow the instructions that came with it to see if there is anything special you need to do before painting it. I would clean the surface thoroughly with a light solvent like alcohol before painting and a coat of lacquer primer before top coating isn't a bad idea either. Otherwise, if you are using an old panel the wax and clear coat on the surface must be sanded down in order for the primer to adhere to the old paint. For large panels (fenders, hoods, etc.), this job can be greatly simplified by using a small, hand-held power sander. A "dual-action" or "multi-action" sander is preferred. Always use water when sanding paint. Wet sanding prevents the sand paper from clogging up and does a much better job of sanding evenly. Dry sanding leaves deep scratches that are very difficult to deal with. Warning: It is very easy to over sand with a power sander so you must go slowly and be gentle. It is particularly easy to over sand on edges and humps, so be extra careful in these areas. Do not sand beyond the primer. The idea here it to keep the factory primer and metal treatments in tact which is the best way to prevent rust. The factory uses several layers of special treatments, often including galvanizing (zinc coating), before priming the panel to ensure that the metal will stay completely in tact. No current consumer product or service can duplicate the quality of the factory paint job. So, if you start to see gray or brown paint through the top coat, you've reached the optimal sanding point. Sand no further. For example, sanding a red or black painted panel will usually reveal gray primer while sanding through a gold painted panel will usually reveal brown primer.
Wipe the surface with a clean towel and fresh, clean water to remove all sanding dust and allow to dry thoroughly. Follow-up with a tack rag just before priming.
Prime the surface with a coat of regular, lacquer primer. Be sure to reduce the primer with lacquer thinner according to the primer's instructions. See the 1st Priming section, step 2 below.

Repairing Rust

Use a scraper, putty knife, or flat-head screw driver to scrap off the rust chips, scales, and paint flakes that are in the rusted area. Be sure to check both sides of the panel you are working to ensure that the panel has not rusted through. All loose rust must be removed from both sides. If you end up poking holes in the rusted out areas, the rusted out metal must be removed. If the panel did rust through, you will have to cut away the rotted-through metal. Using tin snips or a drill and a course file, remove about 1/8" to 1/4" of the metal around the hole so that there is solid metal surrounding it. Make sure that the edges aren't sticking out at all and use pliers to straighten out any kinks made by the tin snips. Be careful not the scratch up or nick the areas that still have good paint on them. If you just have a few pin holes, you don't need to cut-out the metal.
Completely removing the rust from the metal surface is the most important step of body repair, unless you are using special paint that is designed to paint over rust (such as POR-15, which I recommend). If you are, follow the manufacturer's recommendations. Avoid "rust conversion" primers; they generally don't work. Otherwise, take a piece of 60 or 80 grit sandpaper or a wire brush (wire brush then sandpaper works the best) and clean up the metal areas that were exposed in step 1 (on both sides, if necessary). Sand until you see shiny metal and then scrap again with your scraper to ensure the metal is solid metal and not metal over the rust. Keep sanding and scraping until you are down to solid, shiny metal.
Gently sand using a finer sandpaper (150 - 220 grit) and feather out into the good paint surrounding the affected area until you can see the spread of the layers of paint (bare metal-primer-color coat-clear coat).
With a plastic brush, scrub and clean the area with soap and water and dry thoroughly. Dish washing detergent (such as Dawn) works well.
If you are not using a special paint-over rust paint, you must use a rust remover compound to remove the rust from the pitted areas that can't be reached by sandpaper. There are easy to use gels such as "Naval Jelly" or powders that have to be mixed with water such as "Rust Raise". "Naval Jelly" or equivalent can be found at most auto parts stores and is easier to apply because it stays where you put it. Follow the instructions on the package and apply more than once until all of the visible rust is gone, but don't allow it to get onto the painted surfaces that you don't intend to repaint. Mask the area from the rest of the panel using masking tape and paper if you want to. "Naval Jelly" leaves a white powder on the metal after washing it off. Scrub this off with a plastic brush or sandpaper to see the metal below. Use clean 100 - 150 grit sandpaper between applications to make sure no rust is hiding beneath the surface. It is vital that all the rust be removed otherwise it will "crawl" underneath the new paint and you will be back to square one.
When all rust has been removed, sand the surface one last time with a clean piece of 150 - 220 grit sandpaper and scrub and clean the area with soap and water and dry thoroughly. Use a heat gun or hair dryer to ensure the surface is completely dry, and allow the surface to cool for a few minutes. Don't wait too long before applying the first coat of primer because the bare metal begins reacting with the oxygen in the air immediately which starts the formation of rust.

Patching Holes

If you don't have any holes in your panel as the result of the above section, you can skip this section. Otherwise, this section will detail how to patch those holes. If you wish, you can apply a coat of primer before patching a hole. Please read the instructions on your patch kit to see if this is possible. If so, you may want to try using a rust conversion primer here instead of regular primer. I recommend using "Extend" spray-on rust conversion primer. See the First Priming section for more details.

There are various ways that you can make patches. The most common way is to use a fiber glass patch kit which is available at most auto parts stores. I will not go into details on how to use the kit because the procedure can vary. Read the instructions on the package carefully. The patching almost always takes place on the back of the panel and there is usually little surface preparation necessary besides cleaning the surface with a light solvent such as alcohol. Be sure that the patch does not stick out the outside of the panel beyond the surface of the surrounding metal or you'll have a hump. Allow for sufficient curing time before proceeding.
After patching the hole from behind, you'll need to fill the gap in to smooth out the surface. Proceed to the Filling Dents section to accomplish this.

Filling Dents

This section is used if there are any dents in the panel as the result of an impact, or patching holes as in the above section. If you wish, you can apply a coat of primer before filling the dents, unless you have already done so before patching holes. Please read the instructions on your body filler to see if this is possible. If so, you may want to try using a paint-over-rust primer here instead of regular primer. I don't recommend using spray-on rust conversion primers, such as "Extend". See the First Priming section for more details.

If damage from an impact caused the metal to stick out from the original surface further than it should, these section need to be pushed in so that they don't stick out when filling takes place. A light "body hammer" or even a regular hammer and a wooden block will work to push the metal back some. You can always fill a gap with filler, but you can't shave away metal that is sticking out.
There are a few different body fillers out there, but most of them are a polyester filler (such as "Bondo"). They're fairly easy to use, but require mixing (like epoxy) in order for it to cure. Read the instructions carefully. Apply the mixed filler to the dented area with a putty knife so that it extends out a bit from the surface of the panel and allow it to cure. Since "Bondo" uses a hardener, it does not matter how thick the Bondo is, the curing time will always be the same (as long as it's mixed correctly).
Using 100-200 grit, sandpaper dry sand the filler until it is nearly flush with the surface of the panel, but be careful not to sand into the primer if you primed the surface first. A long straight edge, such as a framing square or level, is useful if the area being filled is quite large. Use progressively finer sandpaper as you near the surface of the panel so that the filler becomes smooth. Using 300 grit wet sandpaper, wet sand the area until the edge between the panel and the filler is smooth and the area of the filler is flat relative to the surface of the panel. If you have a flat, very sharp file, you can very carefully cut down any high spots. Otherwise use the sandpaper against a hard, wooden block to shave those spots down. If you used a paint-over-rust primer and accidentally sanded to bare metal, you can try to re-apply the primer, but it may cause the edges of the filler to flare up. If this happens, you'll need to sand them down again and the effectiveness of the primer may be inhibited.
Once the filler is smooth and even, wipe the area down with a clean cloth and water and dry thoroughly.
Small gaps or dents can be filled with "spot putty". Allow the putty to cure and wet sand it down with 300 grit sandpaper.
Replace the masking, if necessary and proceed to the First Priming section (even if you have primed once already).

First Priming

Use this section if you are priming before using a body patch or filler. Once you have filled, repeat the procedure before moving on so that you have a complete primer coat before sanding. Also, if there was any rust involved with patching of filling the body, I highly recommend applying a few coats of primer to the back of the panel in the same areas. This ensures that the panel is sealed from any air and moisture. Even if there was only surface rust, the metal may be able to get oxygen through tiny pores in the back of the panel and rust under your new paint.

If you are going to repair any holes or fill any dents, be sure and read the instructions that came with your patch kit and/or body filler to see if you should prime it first. Some recommend that you apply the patch to bare metal. I use "POR-15" on the bare metal and apply the patch or body filler to that. If the patch suggests priming first and you are going to use a paint-over-rust primer, use a regular, lacquer primer over the paint-over-rust primer before applying the patch. If you need to apply the patch or filler now, go to the Patching Holes or Filling Dents section, then return here.
Priming is the second most important step and choosing the right primer can be a bit overwhelming. There are many varieties of primers out now, many claiming to stop or convert rust. My personal experience has shown that these "rust conversion" primers (go on clear or white and turn blue or black as it "converts") don't work as well as they claim. I think the problem with them is that they react with the rust and iron on the surface, but leave rust underneath and do not effective block air and moisture from it. Completely removing all the rust beforehand seems to help these primers out. A much better product is POR-15, which in my experience has worked very well at halting rust. "Rust-O-Lium" makes some primers that they claim are for "lightly rusted" (white primer) or "heavily rusted" (brown primer) surfaces. I am not sure how these work so I can't vouch for them. It is a good idea to use regular, gray (or brown) lacquer automotive primer (sometimes called "sandable" primer) over any rust conversion/paint-over primers before top coating. This ensures that the rust conversion primer is properly sealed (some appear to be somewhat porous). You can just use regular, lacquer primer as well. If you are using a spray gun and primer from a can, make sure you reduce the primer with the appropriate amount of thinner (see the instructions on the can). Also, most paint manufacturers recommend using the same brand or type of paint for the primer and the top coat you are going to use later. See the Top Coating section. POR-15 has a special lacquer primer that is designed to bond to their paint-over-rust paints. If you decide to buy the top coat from the dealership or a vendor that deals with matching paints, then matching the brand is not possible. I would not worry about that so much unless it is some sort of specialty paint that requires a certain primer to go with a certain top coat. Almost all paints are compatible with lacquer primer. I typically use two primers on areas that are prone to chipping and/or are rusted (front of hood, door edges, etc): I use clear POR-15 on the bare metal and then their POR-15 "Tie-Coat" primer over that after it has dried (about 48 hours). For other areas, I use regular grey lacquer primer (sometimes called "sandable primer") over the factory primer. I avoid sanding down to bare metal wherever possible.
Skip this step if you are painting the entire panel. Mask off the area you don't want to get paint on using masking tape and paper (newspaper works fine). Be sure to use plenty of paper and cover everything in every direction for a least a foot or more. Even though you may not spray directly on a surface that is beyond the masking, spray painting generates a fine, sticky dust that can get on nearby surfaces and can be a pain to remove. So, the more you cover up the safer you'll be. I usually cover everything within about 3 feet of where I am painting. Make sure that all areas with exposed primer and metal is exposed by the masking (you will be extending the unmasked area later to cover the rest of the sanded area).
Wipe the area that is to be painted using denatured or rubbing alcohol on a clean cloth (or paper towel) to remove any oils and residues that may have been left by the soap. Wipe again with a clean, dry cloth or towel. Let the surface completely dry for a minute or so and then proceed to the next step.
Prepare for your first coat of primer. If you are using a spray can, make sure to read the instructions and to shake it well. Spray using full strokes across the area in one direction about 8" to 10" from the surface, over lapping the masking. Always start the stroke on the masking and end on the masking at the opposite side. Repeat in overlapping strokes from left to right all the way down. Don't worry if the first pass doesn't seem like it's covering completely--you'll get it on the next pass. If the paint starts to run, stop painting and wait for this coat to dry before wet sanding the surface again. This usually means that you are either too close, not sweeping fast enough, or you added too much thinner. If the paint starts to ripple, stop painting and wait for the coat to dry before wet sanding the surface smooth again. This usually means that you don't have enough thinner in the paint. Wait for about 20 to 30 seconds and make a second pass the same way. Repeat until you have made between 3 and 4 passes. If your using a paint-over-rust primer such as POR-15, you'll need to follow the instructions carefully (POR-15 uses 2 coats with 4-5 hours of drying time in between). Otherwise, follow the recommendations on the can of primer. Allow the primer to dry for at least 48 hours.
If you need to repair any holes, go to the Patching Holes section. If you used a paint-over-rust primer, I highly recommend that you apply a coat of regular, lacquer primer (or "Tie-Coat" for POR-15) over it after it has cured. Use the same method of application, but be sure to follow the directions on the can of primer. If you already applied a layer of lacquer primer, go to the Second Priming section.

Second Priming

Here, you will apply a smooth primer coating over any dents or patches you may have filling and/or sanded in the last section. Even if you did not use putty, the primer itself fills in small imperfections and the sanding takes down small high spots and ridges. This coating ensures an even primer coat all over the panel effected area.

Wet sand the entire primed area with 500-600 grit wet sandpaper until the surface feels smooth.
Wipe off the surface with a clean, wet rag followed by a dry one. Examine the area and look and feel for any imperfections. Any bumps or high spots can be removed with a sharp file or 300 grit wet sandpaper on a hard block. Any little dents can be filled with spot putty which is available at most auto parts stores. Apply the putty with a small spatula (or a clean finger), allow to dry, and wet sand it with 300, then 500-600 wet sandpaper. Lightly wet sand the surface once more with 500-600 grit wet sandpaper so that the surface is uniform and smooth.
Apply another coat of regular, lacquer primer as you did in step 5 of the First Priming section and allow it to dry for at least 24 hours.

Top/Color Coating

If you are going to be clear coating, then this is the color coat. It is important that the color coat is not allowed to cure for too long a time. Read the instructions on the can for details. If you are not going to be clear coating, then this is your top coat. Since it will have no protection from the elements, it is important that this paint is designed for automotive use so that it will polish nicely and be able to handle weathering.

Skip this step if you are painting the entire panel. Remove the masking around the painted area. Carefully wet sand the new primer around the edges with 300 grit wet sandpaper until it is flush with the original paint surrounding the primer. Now carefully wet sand the entire area that you sanded in the Preparing Surface section with 300 grit wet sandpaper until the whole area is uniform and the seams from the masking and any scratches from previous sanding are smooth.
Lightly wet sand the entire area again with 500-600 grit wet sandpaper until it is smooth. Wipe the area down with a clean, wet cloth and then dry.
Mask the area in again leaving all of the sanded area exposed.
Wipe the area to be painted with a light solvent such as alcohol.
Finding a top coat that matches the original paint is always a nightmare. It is usually best to purchase the paint through a supplier that deals with factory paint codes unless you have a non-stock paint, your paint has faded considerably, or you know that the stock paint doesn't match your paint for some reason (as was the case with my car). Your paint codes can be found on a plaque that is mounted on the panel in front of the radiator under the hood. See your service manual on how to read the plaque. Otherwise, you can take a small part of your vehicle that has has your paint on it and try to match it. If you are going to use spray cans, try to match it to the cap of the can which is usually fairly close to the final color of the paint. When buying automotive paint in a (non-spray) can, you will need the reducer for that type of paint which often contains resins. Once the reducer has been added, the paint must be used or discarded that day, so only mix as much as you'll need. Also, these paints usually require a clear coat over them which is usually of a polyurethane type. See the Clear Coating section below.
Apply 2 or 3 coats of top/color coat to the area using the method described instep 5 of the 1st Priming section. Use the between coat time and drying time giving on the paint can. If your color coat requires a clear coat, then proceed to the Clear Coating section. Otherwise proceed to the Finishing section.

Clear Coating

Skip this section if you will not be applying a clear coat. Even if you did not apply a color coat that requires a clear coat, you can still clear coat using the same brand of clear paint. Make sure they are compatible.

Wet sand the color coated area using 500-600 grit wet sandpaper until it is smooth. Wipe the area with a wet, clean cloth to remove all traces of sanding dust and allow it to dry.
Most clear coats, including polyurethane, require an activator before it can be applied. Once the activator has been mixed in, the polyurethane must be used or discarded that day so only mix as much as you need. It can be further reduced with regular lacquer thinner. Since it is clear, it is often difficult to gauge its viscosity so I would test it on a piece of cardboard to make sure it doesn't ripple.
Apply 2 or 3 coats of clear coat to the area using the method described in step 5 of the 1st Priming section. Use the between coat time and drying time giving on the paint can.

Finishing

Skip this step if you are painting the entire panel. Remove the masking around the painted area. Carefully wet sand the new paint around the edges with 300 grit wet sandpaper until it is flush with the original paint surrounding the primer. Now wet sand all the area that has been sanded with 300 grit wet sandpaper until the whole area is uniform and the seams from the masking and any scratches from previous sanding are smooth.
You may skip this step, depending on the quality of the final painted surface. Wet sand the entire area again with 500-600 grit wet sandpaper until the surface is smooth. Dry the surface off and examine it. The surface should have a "frosted" look to it, but there shouldn't be any underlying scratches. If there are, wet sand those areas carefully with 300 grit and again with 500-600 grit wet sandpaper. When finished, wipe the area with a clean, wet towel and dry. Allow the newly painted surface to cure for a few days before polishing.

Polishing

You shouldn't try to polish new paint until it has fully cured. This can take from days to months. Use polishing compound to polish the surface. Take a clean cotton cloth, put a small amount to polishing compound on it and rub it into the sanded area using circular motions. This step takes a lot of patience and elbow grease before you'll get a nice shine out of the painted surface. It may help to use rubbing compound (brown) on the surface first using the same method of application before using the polishing compound (white). Rubbing compound is more course than polishing compound, so use it before the polishing compound and make sure you don't use the same cloth. Rub the compound in for a while, then wipe it off before it dries on to see what the surface is like. Repeat applications of the polishing compound until the surface starts to feel glassy and look shiny. If sanding scratches begin to appear, you'll have to go back to the Finishing section, step 1 to remove them.
You can bring out the final shine in the paint by using a buffer or a buffing wheel on a power drill. Allow at least one week after the last time you sanded before applying wax to the new paint which will bring out the final luster of the paint.

As with any newly painted surface, the tendency for the non-factory installed paint to fade and oxidize is always a problem. Waxing the surface yearly (or more) and keeping the car in a garage at home is the best way to make the new paint last. The color coat-clear coat system seems to be the most resistant to fading, though it too will fade at a different rate than the original paint, over time.

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Updated 10/30/2003.