This video and post was intended to post on Tuesday, October 30 while I was away at SEMA. The video did upload to YouTube as scheduled, but did not post on this website. There were some server issues with Collision Blast during this time, but we think we have it working correctly now. Hopefully, the videos does not post again as it was intended. If it does, I will delete the second post.
While at SEMA, I attended a class taught by Shawn Collins with 3M. His class further explained this topic and expanded on the problems that can occur if body filler is applied over paint. I have a lot of great SEMA products and training to post over the next few months, but I will get back to the topic for this video and the original post intended.
Bondo Over Paint – Can Body Filler Be Applied Over Paint?
This is a common question I have been asked many times. Whether you’re a “die hard must be to the metal fanatic” or you’re quick to “apply bondo over anything kind of a guy”, you may want to read this or watch the video before your next body filler job.
I was taught that body filler must be applied to bare metal. Therefore, all coatings must be ground or sanded to bare metal before applying body filler. However, I learned auto body repair during the lacquer/enamel days. Since then urethanes started being used on cars and some technicians started applying body filler over roughed up paint. However, if applied over a sanded paint surface, it must be an OEM substrate. This means a factory paint job that has never been repainted. Now we have waterborne paint to mix into the equation.
The problem with this practice is the “technician see technician do”. Someone notices that body filler is being applied over sanded paint, then they assume this is a standard practice and they start doing the same thing. Before you know it everyone is doing this the same way, but many of them do not have all of the facts. The fact the that paint surface must be an “OEM finish” is the missing link, which leads to filler being applied over all painted finishes.
What Problems Could Occur?
So why is there a controversy about if body filler can be applied over sanded paint or not? What could go wrong? Well, I guess it’s like a houses foundation example. You know, the house is only as good as the foundation. Your filler work will only be as good as the coatings it is holding to. So adhesion problems is one of the problems that could occur.
What 3M Said
I have heard both opinions of this for years, but no concrete data. About one year ago I visited the 3M headquarters in St. Paul, Minnesota. While in their lab I asked this exact question. They said that body filler should not be applied over sanded paint. They said that this can cause repair mapping. It was explained to me that many technicians mistake this for sand scratch swelling or primer shrinking. When in fact, the chemicals in the filler can react with the layers of paint coatings causing them to swell or look a look like a shrinking problem.
What Are the Benefits?
So if some technicians have started applying body filler over sanded paint, what are the benefits? The first benefit is that this is much quicker. Less time removing coatings and feather edging. This will lead to more productivity.
Another benefit is that all of the factory zinc coatings and e-coat are not disturbed. These factory coatings are superior for corrosion protection. Regular body filler does not provide corrosion protection. However, newer premium body fillers now have zinc in them to overcome this problem. Therefore, premium fillers can be applied to bare steel and still be warrantied for metals that were zinc coated from the factory.
What Does Evercoat Claim
I did a little research and this is what is on the Evercoat website.
“Our fillers are designed to work over bare, properly prepared substrates such as: steel, aluminum, galvanized, stainless steel, fiberglass, and SMC. Some people prefer applying an epoxy primer over bare substrates to enhance corrosion protection. Our products don’t need to be applied over an epoxy for corrosion protection as long as the bare surface area is clean and no surface rust or contamination is present. However, some auto manufacturers do require body technicians to coat the bare metal surface with an epoxy before applying fillers. If you are performing warranty work, you should consult the manufacturer of the automobile for the recommended procedure. Fillers and putties will normally work OK over properly sanded (80-180 grit) cured OEM paint. However, with so many different types of aftermarket paint available (lacquer, enamel, urethane, water-based). We recommend that all paint be removed where filler is to be applied.”
So What Is My Opinion
There are many opinions on this, but not many clear facts. The I- CAR curriculum I teach and most other textbooks I have used have always taught to remove all of the coating several inches past the repair area to assure body filler will not be applied over paint. I am not claiming this is the only way, but the only way with absolute facts that it will work properly every time. Therefore, this is what I am going to continue to teach to stay aligned with the curriculum.
I also believe this is going to produce the best adhesion for body filler. Here is my thoughts on the topic. Applying body filler over paint may work many times with no problems. However, there is a possibility it could come back to haunt you later. It’s kind of like drinking and driving. You may have done it many times, but if done often enough, you’re likely to get caught.
So, there are the facts and opinions I have, although this is still a gray area. New products are being created everyday! Perhaps a filler will be created and provide data that their body filler can be applied over sanded paint and guaranteed. Better yet, there could be new fillers I am not aware of now. If you know of a filler that has a clear answer for this question and the data to back it, please let me know. I will be interested to learn more and check it out.
One Last Option – Body Filler Over Primer
The only primer that body filler is recommended to be applied over is epoxy primer. I normally do not use this method. However, if you do a lot of restoration work, this may work great. If I had a car completely stripped down to metal or blasted, I would probably go ahead and shoot epoxy over the car to prevent the metal from corrosion. Especially if the car is not going to be completed for some time. However, collision repair that is repaired and filled the say day, this seems like an unnecessary step to me.
Let’s Hear Your Two Cents
You’ve heard the facts and opinions I have, let’s hear yours! Do you apply body filler over sanded paint? Leave us a comment so we can all see how others are doing this.