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How to Prep and Paint a Bumper - Automotive Bodywork Audi A4

Prior Experience: I had lots of rattle can experience, and lots of patience
Cost: no real way to approximate this - depends on what you already have
Time: a weekend

Discuss this mod - Here

I have always been wanting to learn how to do bodywork and paint a car. Ideally I would have learned on some cheap project car - and would have been taught by a professional. Unfortunately things are not ideal. I decided that I was tired of waiting and that I would just teach myself. I got a fundamental understanding by doing some research online and discussing some things with people that have painted in the past.

An audi is probably not the best car to learn on. In particular and audi with a pearl/metallic based paint which is very difficult to match. I realize this, however I decided to take the job on myself. I broke my aftermarket Rieger bumper last year and didn't want to anti up the money to have it repaired and painted professionally. I figured the money saved on labor could be money spent on equipment for the DIY. It was well worth the effort and I am very happy with the results.

Here's how I did it.

All pictures are linked to higher resolutions versions.

Audi A4 bumper paint   Audi A4 bumper paint
Before      After

Part 1: Prepping The Parts

The very first thing you need to do is prep the parts. Prepping is just everything leading up to the paint. In the case of a plastic bumper, it is very important that you take your time and use quality products in the prep work. If you are not careful and clean, your work wont last very long.

PREP SUPPLIES - There are not many supplies needed for the prep work. Listed below are most of them.

Audi A4 bumper paint  
What $$
Sanding Block 5
Total Prep or equivalent (Autozone) 7
3M 5907 Adhesion Promoter (Online) 10
SEM 39133 Primer Surfacer (Paint Supply) 10
Sandable Primer 8
Surface scratch Filler Primer $8
Scotch Brite Pad ?
Sandpaper - 220, 400 ?
 

I will start the writeup with a sanded bumper. My bumper was broken, so I fixed it first, then sanded it down. The original repair was written up under the ABS Plastic Repair writeup. I did it with Duramix products this time, but for all intensive purposes it is the same type of repair. I sanded it until I removed the old rock chips, etc. Unfortunately, there were spots where I broke through to the plastic. You should avoid doing this at all costs as it will make paint adhesion difficult. Do not be too aggressive in your sanding because if you over sand one spot, you will see it very easily once it is painted.

First thing you need to do is take some water and dish soap and clean the bumper VERY well. Use your Scotch Brite pad to get the grime out of the sanded surface.

 

  Audi A4 bumper paint
 

The whole prep process will be iterations of sanding, cleaning and priming. You have to be sure to do a good job cleaning. To help the bumper dry, use an air compressor or a heat gun - or in my case, both. If you are using a compressor, or plan on kicking up a lot of dust, you should hose down the floor of your workspace. This will prevent any dust from coming off the floor as you work.

Dry off the bumper

 

Audi A4 bumper paint

 
Although you have cleaned the bumper, you need to spray it down with the Total Prep. This stuff gets all the leftover grime out. Follow the directions - spray it on, let it sit, but don't let it dry on the surface. Wipe it off first   Audi A4 bumper paint
 

When you wipe it off with your clean rag, you will see how much grime was left even after scrubbing with the dish soap and Scotch Brite.

Surely if you had painted this surface without the Total Prep, the paint would have had poor adhesion.

Repeat this process until your rag comes out nice and clean

 

Audi A4 bumper paint

 
Audi A4 bumper paint  

After the Total Prep step, you need to use your 3M Adhesion Promoter. This is especially important on this plastic part. It becomes even more important when you have made the mistake of sanding down to the plastic like I have.

To use this product, follow the directions: Spray it on, and allow it to dry ~ 10 minutes

 
Audi A4 bumper paint  

After the adhesion promoter has flashed, you can start to spray the bumper with the SEM Surfacer. This product was recommended to me by my local automotive paint supply. It is useful when you have exposed the plastic like I have.

I applied 3 medium/light coats.

First coat pictured to the left. If you click on the image you can see how thin the coat is. You can clearly see the areas where I focused my sanding, etc.

 

Audi A4 bumper paint  

Here it is after I build up my three coats.

 

After the SEM has dried, you can go ahead and start with the primer. Since the surfacer was in a rattle can, I decided to prime with a rattle can as well. I used some standard Duplicolor sandable primer. This stuff works well, and has a medium build. It fills in small pinholes, etc. Don't use it as a body filler!

This picture is a little deceiving because the primer I used was black. This photo is actually after I laid a coat of primer and did my first sanding step (so you can see where I sanded back down to the surfacer). I used a combination of 220 grit and 400 grit sand paper. My bumper had a lot of imperfections, so I took my time with the sanding and building of the primer. I tried to get the surface as even as possible.

  Audi A4 bumper paint
 

In between each of these sanding/priming steps be sure to thoroughly clean the bumper. I would NOT use the Total Prep in these steps - only the soap and water. Make sure the bumper is COMPLETELY dry before applying the next coat of primer.

Here it is again after another 2 coats of primer and some sanding with the 220/400 grit paper

  Audi A4 bumper paint
 
Audi A4 bumper paint  

After I sanded that second time, I went a little too aggressive in one area. It actually sanded down through the primer and surfacer.

 

WARNING: Do not do this next step, its just for education purposes!
Audi A4 bumper paint  

Since I sanded down below the primer, I decided that I should use the adhesion promoter again. Before I put on the adhesion promoter, I decided to clean it REAL well with the Total-Prep. I should have known better. As it turns out, the Total-Prep makes short work of the priming I had just done. It totally ruined all the hard work I just did.

Check out what it looked like

Since I just ruined all my work, I gave up for the day. I came back at it the next day and removed all the surfacer/primer from the previous day. I started over from scratch. Essentially following the same process until I got the bumper primed to the level I wanted
Audi A4 bumper paint  

Here's what it looked like.

Remember, if you can see or feel ANY imperfection now - it will be 10X worse when you paint it. Do your very best to get the surface PERFECT before moving on.
Audi A4 bumper paint   Audi A4 bumper paint
 
After you are happy with your prepped part, you can move on to painting.

Part 2: Shooting/Painting the Parts - Base and Clearcoat

Now that you have put hours into your prep work, you should be able to move onto paint. I cannot stress this enough - If you see an issue with your prepped part FIX IT. It will look 10X worse with a base/clear coat on it.

With that said, lets move on.

PAINT SUPPLIES - The supplies needed for painting vary. It is difficult to say what you need. The big part about this is getting a good gun/air compressor setup. I will spend some time outlining the setup I used

Audi A4 bumper paint  
What $$
Spray Gun Setup ~100
Disposable Gloves 5
A Non-Disposable Respirator Mask 30
Dupont ChromaBase Base Coat , 1 pint 43
Dupont 7160S Base Maker, 1 QT 20
Dupont Nason Clear Coat, 1 QT 24
Dupont Clear Coat Activator, 1 HPT 11
Measuring/Mixing Cups and Sticks 5
 

 

Gun Setup: If you have done any research whatsoever, you will know that one of the most important part of the process is the spray gun and compressor setup. I did some research and decided on a 2 spay gun combination. I only used one on this job since I primed the parts with a rattle can. The setup I bought was the following: For paint and clear coats, I bought the Astro Pneumatic STAR series EVO4014 gun. This is a LVLP (Low Volume Low Pressure) gun with a 1.4mm nozzle. For Primer I purchased the Astro Pneumatic Quantum Ultra Light (QUL107) HVLP (High Volume, Low Pressure) gun with a 1.7mm nozzle. I read a lot of reviews and these guns are very good buys for the money. I purchased the guns online I got both of them for about $150 bucks (on sale from $240). After using the one gun, I can say I am very very satisfied with my investment. You will see the results for yourself in a minute.

Air Compressor Setup: My air compressor is a standard Craftsman 33 gallon, 2Hp air compressor that can do 8.6CFM at 40psi. When you look at your setup, you should cross confirm this CFM number with the gun requirements. My compressor is good enough for this bumper/side skirt job, but would not be sufficient for an entire car, etc. In order to get a good, clean air supply you should have a decent air filtration system setup.

I purchased a 30 dollar air filter that mounts inline with the compressor (shown to the right). I also bought a disposable filter system for the gun - you can see the red thing mounted at the bottom of the gun.

In order to regulate the air pressure at the gun, I bought a small gun mounted regulator as well.

I would recommend not keeping your compressor plugged in while you shoot. This will prevent it from kicking on in the middle of a pass. Monitor the pressure on the gun, then when needed stop painting and run the compressor.

 

  Audi A4 bumper paint
 

I made a ghetto paint booth out of my crappy garage. I hosed down all the walls and floor and put up some plastic to quarantine the work area.

 

Audi A4 bumper paint

 

Setting up your gun is the next important step. In order to see how the gun is going to shoot, mix up some paint.

The Dupont ChromaBase is not cheap, but it is excellent. My car color is LZ9W with is an Ebony Pearl coat. I have had body work done in the past and the PPG paint mix just did not match. When I got work done again recently, and the body shop matched the color perfectly, I found out who their supplier for paint was and figured out that they used the ChromaBase.

Since it was not exactly warm out (60 Degrees) I opted for the Low Temp base maker. To mix the paint, follow the directions. For the Chromabase it is 1:1 Color to BaseMaker

 

Audi A4 bumper paint

 
Lets not be stupid and just start spraying parts here. Wait until the mixing stick dries and confirm that the color matches your car. Obviously it will not have the shine since it does not have clear coat yet - but you should be able to tell without much issue.   Audi A4 bumper paint
 

If you are happy with the color, pour the paint in the spray gun cup. and get ready. PUT ON YOUR RESPIRATOR. I cannot stress this enough. DO NOT try to get away with some disposable mask.

Spend the 30/40 bucks and get a good, non disposable mask. When you use it, you will thank me. You actually cannot even smell paint while you are painting!

 

Audi A4 bumper paint

In order to setup the gun, you need to take some practice shots. The specific gun will tell you the pressure it normally operates at. Every gun has a 2 step trigger. By pulling half way, you will shoot air only, by pulling it the full way, you will start to pull the paint material into the gun and shoot paint.

You want to setup your pressure to read the specified pressure while the is shooting air only. So pull the trigger half way, and set your pressure.

After you have a ballpark of the right pressure, setup a piece of cardboard and hold the gun 8-10 inches away. Pull the trigger half way and hold it until the pressure equalizes, then briefly pull it the rest of the way to see what the pattern looks like.

You are looking for a nice fan pattern with paint evenly spread throughout the pattern, nicely atomized and not runny at all. You have to tweak the pressure and adjustments on the gun until you are happy.

Refer to your guns manual for additional guidance. Here is what a couple pattern adjustments will do.

Audi A4 bumper paint
I went with one of the two circled patterns - I cannot remember which one.
 
Audi A4 bumper paint  

I also recommend that you get a gun holder. This will be nice in between coats - they only cost like 10 bucks.

 

Ready to start painting?!

After you are happy with the pattern, practice spraying on the cardboard. Remember, pull the trigger half way and wait, then pull it full way while you start moving. Move across the part at a constant speed and release the trigger before you stop moving.

Overlap each pass to ensure nice even coats. I would recommend paying additional attention to all the edges. The air tends to blow paint of the edges and you will end up with light coats in those areas. When you sand later, you can break through these parts. This is especially important on the clear coat.

 

Audi A4 bumper paint  

When you are ready, shoot a nice, even, light coat across your part.

Notice: When I shot my base coat, it landed on the part and looked like ORANGE PEEL. However, it is designed to run together as it is drying. This is why your base maker should match your booth temperature. If the paint dries before it runs together, you'll have horrible orange peel in your base coat.

Here's what it looked like immediately after shooting (still wet)

 

Audi A4 bumper paint  

And here is the same part after it has dried. The ChromaBase was incredibly easy to work with. Which made spraying the base coat a quick and painless job.

 

You should go ahead and spray the base on all your parts at the same time. Here is how the bumper was setup and some other photos. (Yes I straightened out the bumper before I painted it). And my buddy stopped by and took an action shot!.. haha

Audi A4 bumper paint   Audi A4 bumper paint
 

Shooting Clear Coat: Now, I should have seen it coming. I cruised though the base coat with no issues. It dried perfectly and I was very happy. So of course, I should have known that the clear coat was going to be a headache. The clear I got (I only found this out after the fact) was not nearly as high quality as the ChromaBase. It seemed to be very thick - and it was not forgiving. I tweaked my gun to get it to shoot the best it could and started spraying parts. Unlike the base, the clear did not run together nicely after it was shot. It stayed like an orange peel. In hindsight, I think I should have put a small amount of urethane thinner in the clear. It is not called for, but I think it would have helped.

Ultimately I shot the clear as directed. Mixed it 4:1 with the activator and went to town. I could tell I wouldn't be happy with the results but I forged on knowing that I would just have to wet/color sand it.

I ended up shooting a couple light coats on one part, and a couple heavy coats on others. In the end, I think that wet coats sprayed on lightly was the way to go (i.e. not a lot of material being sprayed, but spraying it over and over before it dries to build up a nice wet coat).

More or less no matter what I tried I got orange peel. Here are some photos. Sorry that they are blurry - the shutter speed was messed up and I was not trying to waste time with it.

 
Audi A4 bumper paint   Audi A4 bumper paint
 
Audi A4 bumper paint   Audi A4 bumper paint
 

Part 3: Finishing the Parts

Obviously there was no way those parts were going on my car as they were. Since I saw that the orange peel was inevitable, and I knew I was going to have to wetsand the parts, I decided to lay the clear on nice and thick. This way I could sand it and not worry about breaking through it.... or so I thought.

Finishing SUPPLIES - Do yourself a favor and get some good finishing supplies. It will save you a lot of headaches. Here is what I used.

Audi A4 bumper paint  
What $$
3M Sandpaper - 1000, 1500, 2000 grit 20
Disposable Gloves 5
Porter Cable 7424 Polisher ?
3M Super Duty Rubbing Compound 19
3M Perfect-It II 05973 Rubbing Compound 25
Adam's Polishes Detail Spray ?
Adam's Polishes Revive Polish ?
Adam's Polishes Scratch and Swirl Remover ?
 

 

With the exception of the Perfect-It II 05973, I had all of these products from standard auto detailing. The Adam's products and the Porter Cable 7474 Setup were all available through Detailer's Domain , and I got the 3M products from a local Auto Value.

 

Put all your sand paper in a bucket of warm soapy water (dish soap, etc). Let the paper soften up. I suggest the 3M paper because it really works well, it doesn't clog, and it gets soft but never rips. I had a couple other brands around, and they just don't work as well.

  Audi A4 bumper paint
 

The best practice is to start with the least severe technique first to see if it eliminates your problem. I would suggest using the SuperDuty rubbing compound on an aggressive polishing pad. I actually didn't do this, but at the time I didn't realize how hardcore that SuperDuty stuff really is. I bet it would have done the trick.

Regardless, I started with my 1000 Grit paper. Keeping the surface wet and soapy (dish soap), use your sanding block and work the paper across the part in small 4 inch strokes.

 

Audi A4 bumper paint

If there are any shiny parts left - then these are areas you have missed. Make sure you get the whole part sanded and smooth - all of your orange peel gets sanded out. This is why it is important to put EVEN coats of clear EVERYWHERE on the part. Otherwise you will break through when you sand in those light parts.... ask me how I know. Work your way from 1000 up to 1500 grit paper. If so inclined, use the 2000 grit as well.

After you are nice and sanded, take your SuperDuty compound on a medium aggressive pad.

 

Audi A4 bumper paint

 
Spray the apart with the detailing spray and then work the compound across the part without the Porter Cable turned on. This will spread it evenly and prevent splatter and wasted products.   Audi A4 bumper paint
 

With the PC on about 4, work the polish into the part evenly. Do not stay in one spot too long or you will risk breaking through the clear.

 

Audi A4 bumper paint

Wipe off each product before it dries hard. I worked my way from the SuperDuty, to the Perfect-It to the Revive Polish with scratch remover all the way up to waxing. Note: You may need to wait a period of time before waxing - check with the paint manufacturer.

The most impressive product is the Perfect-It II. This brought a pretty nice shine out from parts that were just sanded with 1500 grit. The Adam's products then finished off the job.

 
Audi A4 bumper paint  

Side Skirt half done

 

 
Audi A4 bumper paint  

Wet sanded bumper

 

After Perfect-It Step

 

Audi A4 bumper paint

Polished and waxed. This is ready to be installed..... Better pictures to come!

Audi A4 bumper paint

 

Audi A4 bumper paint

 

Hope you give this a shot and find the writeup useful.



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