Lesson 5 – Body Filler – Part 1

This is a common question I have been asked many times.  I do not recommend applying body filler over sanded paint, but this subject has been controversial through the years.  I do not want to confuse you, but to educate you with more ways than just mine.

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 have been used on cars, and some technicians have started applying body filler over sanded 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 sees 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 lack all the facts. The fact that the 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 whether body filler can be applied over sanded paint or not? What could go wrong? Well, I guess it’s like a house’s 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 are one of the problems that could occur.

What 3M Said

I have heard both opinions of this for years, but there has been 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 appear to shrink.

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 remain undisturbed. 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 warranted 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.”

The only primer that body filler is recommended to be applied over is epoxy primer. I usually 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 apply epoxy to the vehicle to prevent the metal from rusting, especially if the car is not going to be completed for some time. However, collision repair that is repaired and filled the same day, this seems like an unnecessary step to me.

I have a video on this topic.  Watch the video, then we’ll move along the body filler process.

Use a nonporous surface and a plastic or metal spreader to mix your filler. Do not use cardboard to mix filler on, as cardboard will absorb some of the resins in the filler. You can use plastic, steel, or any other surface that is not porous. Next, open the can of body filler. If it is separated, it will need to be mixed. You can do this by using a paint stick. The cream hardener will also need to be kneaded. You can do this by squeezing the tube with your fingers, which mixes the hardener inside the tube. Next, put the amount of filler needed on the mixing board. For every golf ball size of filler, use about a dime size of hardener. Now you can mix the two parts together. Do not stir or whip when mixing. This will cause air bubbles, which will lead to severe pinholes. Mix the filler in one direction by folding and spreading in one direction. Be sure the filler is mixed thoroughly until the filler is one uniform color. If you can see streaks in the filler, keep mixing.

Apply The Body Filler To the Repair Area

Now you can use a plastic or metal spreader to apply the filler. You only have 5 to 10 minutes to apply the filler before it starts to harden.  If it starts to harden on you, stop, as it will only make a mess when spreading.  Apply the first coat on tight by pushing your spreader hard against the surface. This will push the filler tight into the ground scratches, producing superior adhesion. It is better to apply several thin coats, rather then trying to fill it in one thick coat. Once the tight coat is applied, follow the tight coat with a fill coat by releasing some of the pressure when spreading the filler.  Sometimes one application is enough, but don’t be afraid to apply a second or third coat if needed. However, as mentioned, don’t exceed 1/4 inch of combined layers of filler thickness (after sanded.) Be sure to sand between coats to identify low areas. If there is leftover filler on your mixing board after application, it must be thrown away. Never try to put leftover filler back in the can, as this will ruin the entire can of filler.

Skim Coat

A skim coat is a final coat of body filler applied to the entire repair surface.  Not everyone uses this method, but it helps make that invisible repair.  Throughout the body filler stages, you may have several layers of body filler feathering into other layers of body filler.  Sometimes you can feel the different layers when feeling it with your hand.  One way to assure everything is level and ready for primer is to spread a skim coat on the entire repair area.  This is a thin coat of body filler that is applied, much like a tight coat.  Spread the filler over the entire repair area, but do not allow the body filler to spread onto the top of painted surfaces.  Once dry, block sand the whole surface to produce a level surface for priming.

You can also do this with finish glaze, which we will discuss below.  Another trick for applying your skim coat is to add a little liquid fiberglass resin with it.  To do this, add some body filler to your mixing board.  Next, pour a little resin, about the amount of cream hardener you usually use, of the liquid resin.  Then, use regular body filler cream hardener to mix the filler as you usually would.  This will make the skim coat thinner and spread more easily, and help the skim coat go on smoothly.  Allow to dry thoroughly, wipe the top of the filler surface with some lacquer thinner on a rag to remove the tackiness created by the resin, then block sand.  This is a trick that has been around for a long time.  Experiment with it and see if you like this method.  As mentioned, finish glaze works well for this step as well and has a few more benefits we’ll discuss below.

Detecting Body Filler

So, how do you know if there is body filler on a vehicle if it’s covered with paint?  In the past, filler could be detected by using a magnet.  However, the magnet test does not really let you know if there is too much filler or not.  If the maximum thickness is only supposed to be 1/4 inch, how will you determine this?  Well, there is a new tool to detect the thickness of body filler.  This tool may be useful if you are looking to buy a used car or when estimating a job to determine what you are getting into.  This tool will only work on metal surfaces.  If the part is composite, it will not work.

Fiberglass Filler – What Is Fiberglass?

Raw fiberglass comes in a soft fabric like material. When saturated with liquid resin and hardener, it becomes hard and very strong. There are not too many fiberglass auto parts on modern day cars, as they have all started using other composites like SMC and carbon fiber. However, fiberglass was used on early model corvettes, truck hoods, and many other parts. There are still aftermarket parts that are manufactured from fiberglass and it is still used for boats and jet skis.

The Difference Between Fiberglass and Fiberglass Filler

Fiberglass filler comes in a can and is mixed with a cream hardener. It mixes much like regular body filler, but it is thicker and a little harder to mix. The filler has fiberglass mixed in it. It comes in short hair and long hair. This is the length of the fiberglass that is mixed in the filler. Both provide excellent waterproof properties as they do not absorb moisture. Both fiberglass fillers are stronger than regular body filler. The long hair filler provides the most strength out of the two. However, these fillers are very difficult to sand. The filler is also thick, which makes it hard to level and smooth like regular body filler.

Why Use Fiberglass Filler If It’s So Difficult To Sand?

The reason we use fiberglass filler in auto body repair is not really for the added strength, but for the waterproof properties. It is recommended to apply a thin layer of fiberglass filler over any welding that is performed. Body filler absorbs moisture, which will lead to corrosion and rust. By using the fiberglass, we eliminate the moisture absorption problem. Since our main purpose is to seal the welded area, the short hair fiberglass is sufficient for the application. 

Lead

Lead has been around for many years.  It was used as one of the first fillers to fill imperfections on car parts.  There are still a few custom shops around that use lead, but most shops have started using lead-free solder or body filler.

If you are a lead fanatic then you probably know the name Bill Hines.  He has been working with lead and became know as the leadslinger and king of leading.  Bill was born in 1922 and starting using lead in 1941.  He has customizing cars since the 1040‘s and it is said that he still works 7 days per week.  This means lead has been working well for many years and still being used today for the right applications.

Lead can be used a s a filler to fill imperfections and to seal welded seams.  Lead is not used much anymore due to the health hazards of lead as it has been outdated with plastic body fillers.

A new lead-free version body solder has been developed if you are a lead enthusiast and want to produce the same quality results.  The lead-free body solder is used much like the older leads.  The metal still requires to be tinned, solder applied with heat and smoothed with wooden paddles, then shaped using a body file.  You can also sand or grind this solder without the worry of breathing harmful lead particles.  While this is a safer leading alternative, a proper mask should still be worn as when sanding any type of filler.