Cars in the 21st century have gotten much better at many things, but they still get dings and dents from parking lot altercations and garage mishaps. The motor may make more power, put out less pollution, and last twice as long as the once did, but sheet metal has gotten thinner and easier to ding. GM experimented with plastics with the Pontiac Fiero, most of the Saturn line, and fenders on certain cars, and the new F-150 is mostly aluminum, but for the most part body panels are still steel. Steel is easily welded, and a handheld stud welder and slide hammer can quickly pull a body panel back into line.
In the past we have talked about rust repair, which has also gotten less common, and fixing deep scratches in the paint, but what if there is an actual dimple or crease?
The Jeep Patriot we tore down and reassembled for our recent repair manual had a dented front fender from a parking mishap. To fix it we did need to purchase a stud welder and slide hammer kit, which is about $100. Though it would be easy enough to unbolt and replace the fender we repaired it as a demonstration, and will be posting a video or the process eventually.
You’ll need the following materials for this job...
- Stud welder and studs
- Slide hammer
- Angle grinder and flapper wheels
- Body filler
- Various grade sandpaper (80, 400, 600, 1000, 2000)
- Sanding block
- Primer and base coat paint
- 2k clear coat paint (available in aerosol cans)
- Masking materials (paper and tape)
- Rubbing compound
- Mineral spirits
- Tack cloth
1) Remove Paint - In order to use the welder you need clean, bare metal with no paint on it. After putting on proper eye protection and leather gloves, use the angle grinder and a flapper wheel to strip the paint from the area of the dent. In this picture the dotted red lines are to indicate where the dents are that need to be pulled out. The bright green line is a body line that you need to be careful not to ruin by pulling the area around it out too much.
2) Weld Studs - Following the crease lines, weld studs every few inches in the deepest part of the dent. For larger dents, you may need to pull not just the crease, but the less indented area around it as well. Metal stretches slightly, like pizza dough, when deformed and bent, but minor collisions shouldn't affect it too much because it is already so thin. Be careful when welding that you don't inadvertently cause a fire because of undercoating or old dead leaves behind the panel.
The stud welder is easy to use: Stick stud in the end of gun. Press gun to panel. Hold button on welder for 5 seconds.
3) Pull Dents - The end of the slide hammer is sized specifically to fit the studs, with a small thumb wheel that rolls down and grabs them. Slide it onto the stud furthest from the center of the dent, and hammer it back. Work your way from the least dented area to the deepest dent. Depending on the damage, you may need to repeat the process as you work to pull the panel flat again.
4) Cut Off Studs - When the panel is back into the proper shape, cut the studs off with a pair of wire cutters. Use the angle grinder to grind off the residual welded parts of the studs, being careful not to gouge the panel.
5) Body Filler - If you did a good job with the studs and the puller, you should only need a small amount of body filler. Wipe the bare metal clean of all dust and grease before you begin with mineral spirits and a lint free rag. Some people prefer to use a good epoxy primer on the bare metal to help fight rust, but body filler on bare metal is a stronger bond. You may have to repeat this step 3 times or more until everything is flat.
Mix a golf ball sized dollop of body filler with a pea sized portion of hardener; typically the official mixing ration is about 50:1 but it is better to use too little than to use too much. Spread the filler on the panel, forcing it back into holes and indentations, bearing in mind that the smoother you can make it now, the less shaping and sanding you will have to do. Ideally there won't be any area with filler more than 1/8" thick. Remember to only use as much as you need, and take off as much as possible by shaping and sanding.
6) Shaping and Sanding - Start with a coarse grain sandpaper (you can go as rough as 80 grit for initial sanding) mounted in a small flat sanding block to begin initial shaping. You do not have to wait until the filler is fully hard to begin shaping it, and the work will go much faster if you don't.
7) Prime and Wet Sand - Once you have the panel as smooth as possible it is time for a coat of primer or two before the paint. Primer seals the surface below it and promotes adhesion of the paint, plus gives you a uniform surface color so you can use less paint. If you still have some scratches or surface irregularities, several rounds of high build primer and sanding with 400 grit paper will help with the smoothness. If you have little pits and pinholes on your body filler, you may need a tube of glazing putty which is like body filler but thinner and lighter.
You should wet sand with 400 grit all over before priming, especially where the old paint meets the bare metal. Wipe down the surface with mineral spirits to remove any oil or dirt, then use a tack cloth to remove any dust (wipe the surface with the tack cloth again before any coat of paint or primer). Spray primer in smooth back and forth motions, laying down 2 or 3 very coats. Allow the primer to dry and wet sand again with 600 grit paper to eliminate any sanding swirl left in the paint, filler or earlier primer. If you want a smooth, glossy top coat you need to make the primer as smooth as possible first. If you sand through the primer, spray another coat, allow to dry and sand lightly again.
8) Base Coat Paint - Once the primer is perfect, it is time for the color base coat. If you are working with aero spray cans, which can often be special ordered to match any factory color, the addition of a $5 trigger and handle can make getting a consistent finish easier. If you are spraying a metallic, or candy color, which many factory paints are, it is vitally important that you shake the can every few minutes, spray with the can perfectly square with the panel, and move the nozzle with a consistent speed. If you pause for a second in one place you will leave a darker spot, with more metal particles in it, and it will stand out. Spray several thin coats until you get complete coverage using the minimal amount of paint. You may have to spray several more thin coats in order to get a proper match with some paints. This is the hardest part of the whole job.
9) Clear Coat - Nearly all modern vehicles use a two part, base coat/clear coat paint system, with the base coat needing a clear over the top of it to produce the proper sheen and protect it from UV rays from the sun. After the color coat has dried but before it has dried too much, add several thin coats of clear over the paint to protect it and add a higher gloss. Read the instructions on the paint can for the base coat and there will be a time window, often something like "Apply clear coat 2 hours to 24 hours after base coat for best results". If you wait too long the clear coat will not bond well, but scuffing it with 600 or 800 grit paper will help if you have to wait several days before applying clear.
The newly formulated 2K aerosol clear coats allow factory durability without professional spray gear. From the outside it looks like a regular spray can, but in the bottom there is an activator button. Pressing the button releases the activator into the can, which than hardens like an epoxy in a matter of hours once applied. The downside is that these paints can cost more than $20, and the can must be used up within 36 hours. The biggest downside of these paints is that they do contain isocyanates, so you will need a real respirator, and you should cover your eyes, arms and hands too. These are nasty chemicals that can cause serious irritation and will harden inside your lungs if you breath them.
10) Buff Out - Let the paint and clear coat dry for several days (a week would be best), then use a very fine wet sand paper to smooth any "orange peel" in the paint. After sanding with 1000 or 2000, depending on how rough it is, move on to buffing with rubbing, and then polishing compound. When finished buffing it is time for a thorough cleaning of the old paint on adjacent panels and a fresh coat of wax over the whole area If you have done everything properly, for $100 in materials (not including the stud welder) you won’t be able to tell where the repair was made.