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Using body filler, by Reed Overson Last updated on September 28, 2013

Reed Overson lives in Bismarck, North Dakota, and has been a bodyman, painter, sales rep, and technical rep for thirty years.

Reed used to work as a technical sales rep for the company that makes Rage filler, and knows the chemists that developed the resin for it.

The following article contains some of Reed's thoughts on using body filler.

The resin used in Rage and other body fillers is fiberglass (polyester) resin. Today's resin technology makes the adhesion of these new resins as good as or better than most epoxy adhesives. They are designed to really stick. Also, they are somewhat flexible when used properly.

Most, or probably all, filler manufacturers design and recommend them to be used over clean and prepped (sanded with coarse 36-grit), rust-free (sandblasted), dry (free from moisture), bare metal. They will stick to epoxy primers with mechanical adhesion. Body fillers do not chemically bond with epoxies. If you put filler over epoxy primer, it has to be fully cured, with no solvents such as thinner or reducer left in the primer, and still should be sanded with a coarse grit and cleaned. In other words, using epoxy primer under filler is a wasted step that may or may not cause adhesion problems later on.

One of the most critical points is for everything to be clean and moisture-free.

Body filler resin is a thermal-set plastic. It cures with heat. The hardener MEK peroxide is a catalyst that produces heat in the resin. It needs to be at 64 degrees Fahrenheit minimum to cure, and the best is between 72 and 80 degrees Fahrenheit. (If you live in a warm climate, read on, but it may not apply to you.)

If the metal is cold, the filler will cure from the outside in, and could trap some moisture and solvent from the resin underneath, causing adhesion or bubbling problems later. (Styrene is the solvent in resin and produces the smell that we associate with Bondo and fillers). Also, moisture (humidity in the air) will condense on the surface of the cold metal. This is the same effect you get when the mirror in the bathroom fogs up when you shower. The mirror, and the metal of the car, are about 10 to 15 degrees cooler than the air temperature, and the humidity condenses on them. If you warm the mirror or the car's metal to about the air temperature, the moisture won't condense on the surface. Using a heat source to warm up the panel (but not too hot) will eliminate the moisture being trapped under the filler (which can show up as rust under the filler) and help keep the cure temperature even through the filler. This is the way the chemists designed the product to work. You can use extra catalyst to speed the cure, but too much will cause too much heat in the reaction and crystallize the resin which causes it to be brittle and break down, causing more problems later. If you head to the auto body supply store for more hardener you may be using too much and looking at problems down the road. Pre-warming the metal and keeping the shop warm will let it cure more evenly with the correct amount of hardener. Heat lamps or lights should be far enough away so the panel feels warm when you put your bare hand on it (not hot).

The correct amount of hardener is 1 1/2 to 3%. Here's an easy way to figure it:

On the pallet or board that you mix your Bondo or filler on, put the amount you are going to mix up in the form of a circle. With your spreader or mixer divide the circle in half, 50%. Divide the half in half, 25%, half again, 12.5%, half again, 6.25% half again, 3.125% (this is the maximum) half again, 1.5% (this is the minimum).

If you have a cold shop, put a small batch on a warmed test panel to check the cure time, spread to about 1/4-inch thick. It should be fully cured in about 30 minutes to an hour, depending on the temperature and hardener amount. (Yes, I know you can add a pile of hardener and get it to kick in a few minutes.) A good cure time lets the gasses come out, and gives ample time to work out the air pockets with the spreader, eliminating pinholes. After a few times, you'll be able to gauge the amount of hardener for the size of the filler batch that you are mixing.

There is some room here for a little extra hardener, but too much extra. Remember, this needs to work with the temperature of the air and metal. Filler also contains talc (the mineral in talcum powder), which will absorb moisture. If you try to fill holes in a body panel, moisture can be absorbed from the back side, causing the area to swell and bubble. Also, bare filler left in the weather (especially rain) for any length of time can absorb moisture and cause problems later.

It will cost a lot less to follow these steps to correctly prepare the metal and work within the correct temperature range and hardener amounts than to even bother with epoxy primer as an underlayer.

Between coats of filler, do not wipe with solvent (thinner), because solvent will be absorbed into the filler.

Rough-up the areas not sanded and blow the dust off well, and then apply a new layer. The thickness of the layers should not be more than 1/4 to 3/8 of an inch. If you put filler on too thick, the heat in the reaction will be concentrated and higher in the thick area and could lead to crystallizing the resin in that spot, making it brittle and subject to cracking later on. Also, too thick can mean you won't be able to get all of the air pockets smoothed out.

Use a primer surfacer system that is recommended for the paint you will use. Self-etching primers usually contain an acid and should not be used under filler, because the acid will slow down the cure of the filler resin. I like these primers for bare metal on my projects. I use them, but grind them off where I do filler work. Also, the acid in the self-etching primer activator will affect the epoxy resin hardeners in epoxy primers where they may not cure chemically, but will still air-dry. It is possible to put epoxy primer over self-etching primer, but only if the acid is gone, and the etch primer is fully cured. On old cars, my preference is etch primer on bare, sandblasted metal: Bondo or filler on bare metal, a catalyzed spot or glazing putty (Bondo skim coat), a good two-part urethane primer filler, and a good topcoat finish.

Today's Bondo/body filler technology will adhere to properly-cured paint and primers if they are absolutely clean and dry. They stick by mechanical adhesion. They grip to scratches, so the rougher the surface, the better the grip. Solvents (thinners and reducers) in paints and primers can work under the edges of the Bondo/body filler area, and it's easier for this to happen if you have paint or primer under the Bondo/body filler. Even when priming over filler on bare metal, don't put so much on that you have the solvent affect or lift the edge of the filler. Use nice even coats and let the solvents flash (evaporate) between coats of primer.

If you are beginning, do some test batches first. If what you put on the car doesn't look like it's right, grind it off and start over. Body filler is the least-expensive part of bodywork. It doesn't make sense to keep going over a bad mix.

It takes time to learn technique. I've worked with guys that made Michaelangelo look like a rookie when it comes to sculpting. (It usually takes me longer to get the shaping the way I want it, but I do okay.)

Catalyzed glazing putty is basically a finely-textured Bondo. It has less porous filler material added to the mix, so it doesn't absorb solvents as much. Rougher scratches than from using 320-grit would be better, but that will work. The key to using this or any catalyzed primer filler or spot putty with a hardener is to remember that if you put it over a finish (either paint or primer) that does not have a hardener, solvent or thinner will soak into the uncatalyzed product, and not the catalyzed product. The uncatalyzed product will swell a little from the solvent it absorbed and later (usually after you have painted the car). It will evaporate and shrink back down leaving sand scratches or a ring around the area. It's best to use a system with products that are two-part (catalyzed) with hardener from the metal work to the finish. There so many good fillers, primers, glazes, and paints that are two-part systems today.

Back in the day of lacquer paint finishes on custom cars, painters would color-sand the paint after it dried and repaint to get all the sand scratch shrinkage out. This process was repeated until the paint didn't show the sanding marks. They used to say, "This car has 20 coats of hand-rubbed lacquer." "Hand-rubbed" was the term in some areas for color-sanding by hand. The thinner in the paint was stronger, and took longer to evaporate than the thinner used in the primer. When you painted lacquer paint, the thinner would soak into the primer and cause it to shrink back into the scratches in the body work. That is why the process was repeated until scratches didn't show in the final coat.

Concerning thickness, 1/8 of an inch is usually no problem for filling. I've seen some really thick sculpting in my day. Building up an area in layers of 1/4 to 3/8 of an inch will work. The thicker the filler, the greater the chance of having problems down the road. I have seen cars with filler over two inches thick that lasted for years, and some custom cars have had areas that were done even thicker.

If you have any cracks in the metal under the filler, they will have to be welded first. Any places under the filler that can move will show up later. Body filler will have the ability to flex some, but a crack in the panel will eventually crack the filler. Grinding with 80-grit will work, and coarser (36-grit) is okay. Remove all of the old filler and start over using the process I described. If you have trouble getting areas ground out, use a wire brush in a drill or even a broken 1/8-inch twist drill to get into tight areas to grind down to the bare metal.

Preventing rust (or moisture) from forming under the Bondo/body filler repair is a big concern. In humid and cool conditions I don't like to start exposing (grinding) the panel bare until I'm ready to do the filler work and can complete the area to the priming stage. When working on a rusty, old car, to get it down to bare metal, I plastic media blast if possible (or carefully sandblast at low pressure), and then grind just prior to doing the body filler work.

Keeping the panel warm will keep moisture from forming on the surface and help keep the curing process of the filler even. Temperature is the most critical factor when working in cool humid conditions. There are two basic primers to protect from rust. Rust-inhibiting primers seal the area from moisture but do not chemically react with the rust. Etch primers contain an acid (usually phosphoric acid) and chemically etch the metal (the acid converts the iron oxide, meaning rust, to iron phosphate) and the filler part of the etch primer seals out the moisture. Phosphoric acid slows down the catalyst (hardener) in the Bondo/body filler and can make it not cure properly. Today's body filler/Bondo technology basically does the same job on bare metal that a rust inhibiting primer would do. It bonds tight against the surface of the metal (with mechanical adhesion) to seal out air and moisture. Actually the technology in the Bondo/body filler products we use in the US came from Europe so you have had them for about eight to ten years longer than we have. There are no chemical reactions with the body filler (polyester resin) and rust. You could still prime over a small layer of rust, put Bondo/body filler over it and end up with problems later. I use etching primer on bare, sandblasted metal, but I grind it off in the area I do my filler work. I try to keep the area warm, clean, and dry.

You shouldn't use cardboard for mixing filler because you don't know where that cardboard has been or what it has absorbed. I use a product called Clean Sheets, which is a pallet with a tablet of non-porous paper sheets that I can tear off and throw away when I'm done. Most cardboard is porous and will absorb some of the solvent in the resin (styrene), but most of all, the styrene will let loose any chemical that was already in the cardboard, whether it was a box that held oil, grease, or worse, silicone, as is found in Armor All. I once saw someone mix filler on part of an Armor All box. When he went to sand it with an air board sander, it peeled right off. And that's not to mention that he and everyone else in the shop had fisheye problems for about three days. Lexan, Plexiglas, or an old car door glass works far better than cardboard.

It's not the best situation to leave the body and filler areas bare for so long, but with projects it is sometimes the only way. There are two basic self-etching primers. Vinyl-wash primer is a thin, vinyl resin-based primer with a phosphoric acid activator/reducer. It is translucent and mostly used for bare metal only (not over filler/Bondo) and is usually primed and painted very soon after it is applied (wet on wet). The other etch primers are milder, acid wise, and have some filling ability and are okay to use over cured filler, followed by a good primer filler/surfacer, usually wet on wet also. Check with the manufacturer on times and which systems (primers) to use. If your project has been bare in humid conditions, there is a good possibility of a rust film forming on the exposed metal. A scuffing and cleaning followed by the milder etch primer would be my choice. Unless you have a real wet climate, the area under the filler should be okay. I would suggest letting the body be warm and dry for a few days before priming to help let any moisture that may have absorbed into the bare filler areas evaporate. Most primers have some alcohol in the solvents which will help draw some moisture out.

Etching primers chemically grip to the metal by converting the oxidation (rust). They also do this on aluminum, and help in getting the best adhesion on stainless steel. Metal prep / metal conditioner uses the same acids to neutralize the rust. You can have an adhesion problem if you use metal prep followed real soon with an etching primer. The etch primer won't have enough rust (it doesn't take much) to bond with and will not stick. I have seen this happen. I prefer to sandblast the metal then use etching primer (I'm talking old cars here). It's a lot less messy than using metal prep and you get a good chemical and mechanical bond. Follow this with a good quality two part primer filler and you have a great foundation for your topcoat finish, whether it's going to be suede or shiny.

You can put too little hardener in the mix and it won't have enough to full cure the whole mix. Temperature is the key. If it's 90 degrees Fahrenheit out or hotter you will probably have to do the work in the shade and work on smaller areas in order to keep up with the fast cure. You can only slow it down so much or speed it up so much, it has a limit on each end. If you ruin a batch, throw it away. It's cheap stuff compared to ruining a paint job down the road. One thing I noticed the one time I was in Florida is that the body shops had huge air conditioners. We only have huge heaters up here in North Dakota. I'm sure it's common where you guys live to have an air-conditioned even in your home workshop. The temperature is directly proportional to the amount of hardener and it just takes time to get it dialed-in. Also always mix the batch as thoroughly as possible. Not so many years ago our neighbors in Canada used clear liquid hardener for their Bondo/body filler. That is really a pain to use when you can't see the color change like with the creme hardener we use in the States.

I'm going to mention types of primers and not so much specific brands because everyone will have different brand names available in their area. I'm a firm believer in the two-part primers. There so many kinds of them, and I'll try explain the basics of most of them. Urethane-based two-parts are probably the most universal. They are based on resin that uses an isocianate hardener, so there are some health concern here. Most epoxy primers are two-part, based on epoxy resins. Polyester primers are based on polyester resin (fiberglass/Bondo resin) and use the same chemical as a catalyst as fiberglass and body filler. These primers all bond with mechanical adhesion. They grip to scratches in the surface of the panel (metal, fiberglass, plastic, Bondo, etc.) Self-etching primers use an acid-based activator/reducer to convert with the metal and form a chemical bond as well as mechanical adhesion. For resistance to moisture both the type of resin and the amount of solids (filler material) in the type of primer are factors. A good two-part resin that provides good mechanical bond along with dense filler material that does not absorb moisture will be better as a rust inhibitor. There are some really good systems available in everyone's price range. You should do some research and ask questions about the system you are interested in and make sure to get thorough product information reference sheets to insure you use it correctly. In the product information for any particular primer, it will tell you what kind of surfaces it can be applied to, how the surface should be prepared (sanding/cleaning), how much it fills per coat (mill thickness), what chemical or solvent resistance it has, and what topcoats should be used. Read these and ask questions if you don't understand. Most companies have a web site or an 800 number you can reference.

You will find a huge variety of primers for filling, sanding, sealing and protecting. Another thing I really like is POR-15. Sealing the backside of your panels and neutralizing corrosion on the inside will make the repairs last so much longer. Moisture can seep through tiny pinholes and ruin your body filler repair or primer and paint. POR-15 together with a paint or under coat can probably seal most tiny pinholes in the panel. I don't know if it is possible to get them all but this is a good way to start. In areas where panels have been welded I like to seal the inside of the weld with POR-15 and maybe a layer of seam sealer and paint. One thing on DP primers. If used as a rust inhibitor read the tech sheet to see the mills (thickness) per coat. These primers are usually used for tie-coat sealers and should be top-coated with another product. For use as a storage primer you may need several (more than two) coats to get good moisture protection for an extended time. As I remember, the directions on both the Loctite and POR-15 say it is okay to put body filler/Bondo over it after it has cured. You should be doing everything according to the specifications.

Back to primer and rust. Earlier I mentioned pre-warming (not hot) the panel in cool humid conditions (which you and I both share because of climate) to evaporate the moisture that condenses on the metal. The metal will be 10 to 15 degrees Fahrenheit cooler than the air and moisture will condense on it. If you put filler on a cold panel in humid conditions you trap a thin film of moisture under the filler. That is one of the causes for rust under the filler. This is the easiest to avoid by simply warming the panel to slightly above the air temperature (which should be above 70 degrees Fahrenheit.) Grinding or sanding the panel just prior to applying the filler will also help. When spreading the first batch of filler to the bare metal, extra pressure should be used to force the material into the metal as thoroughly as possible. This helps the mechanical adhesion of the filler to the metal. An even cure temperature until full cure is reached will also help with expelling solvents and humidity from the filler as it cures. You are on the right track about primers. The material most used for rust inhibiting and protection against moisture is zinc-chromate. This material has a large molecule that is dense and when used as a filler material in primers helps stop moisture from penetrating. There is a health concern with chromate-based products and we are starting to see chromate-free products. Lead-based paints give the best protection but due to health concerns they are being replaced with other less dense materials. Most old and probably new ocean vessels are painted with lead based primers and paint to ward of the corrosive effects of salt water. When you are buying primers compare the weight of the can. High solids primers may weigh up to 5 pounds more per gallon. Always read the information sheet to see if what you are buying is right for the job you have planned. I like self-etching primers on my sandblasted (old car parts) and yes, sanding the sandblasted steel to smooth out the high spots is a good idea. The etch primer's activator (acid) chemically bonds with the rust to form iron phosphate. If I'm not doing body filler work soon I leave this primer on for protection. For really long-term storage I use a dense urethane two-part primer on top (wet on wet) per manufacturer's directions. When this is fully cured, I prefer to grind the areas that I am going to fill with body filler right down to the bare metal. I could actually put filler over the fully-cured primer system (after sanding and making sure that it's clean), but I usually find some more metal finishing I can do, and it's easier to grind that area off to do the work. If I have areas that are bare for a time in between work sessions and surface rust seems likely, I use a small sandblasting gun to re-clean the areas. I hope this information helps, but everyone reading this has to do some research themselves on the specific products and systems they are going to use.

The DuPont Variprime has an acid-based reducer/activator. You should not put the Rage over the Variprime if there is any chance that it is not fully cured, and the acid has not completely converted. This is hard to determine in cool and humid conditions, which slow the process down. I like to let it dry. Then, before I apply the filler, I remove the Variprime by grinding the area first. This also gives better mechanical adhesion for the filler.

Another option is a rust encapsulator like POR-15 or similar product. Clean or sandblast the rusty areas then apply the POR-15 (or similar product) let it dry per product information and rough the area up and apply the filler. Anytime you apply filler over something that has a solvent (thinner/reducer) base it has to be dried and cured enough so all the solvents are evaporated and still roughed up with abrasive. There are solvents in the filler that could soak into the layers underneath and cause shrinkage later. Self-etching primers (acid-based reducer types like Variprime) are still soluble and can absorb thinner or solvents. Thin layers, maybe 1 coat, will be less likely to cause problems. Thick layers, like multiple coats, can act like a sponge to soak solvents which can remain until after the filler cures, possibly causing problems later on as they try to escape. So even with this information, the best is still to apply the filler directly on the cleaned (sandblasted, wire-brushed, or abrasive-ground), warm, dry bare metal.

Sandpaper grits are largely a matter of preference. Here are some basic guidelines:

  • 16 to 36-grit for grinder discs for paint removal and bare metal Bondo prep

  • 24 to 36-grit for roughing out the filler with an air file sander or hand board

  • 60 to 80-grit for smoothing out the rough scratches from the previous operation and shaping finer contours

  • 80 to 180-grit for further smoothing in preparation for high-build catalyzed primers (with hardener), or 240 to 320-grit for lacquer-based (non-catalyzed) primers

  • 320 to 600-grit for final sanding prep before paint, following the manufacturer's guidelines on their product information sheet

  • 600 to 2000-grit for sanding clear coats for re-coating clear, or for removing minor surface imperfections prior to polishing and buffing.

    The last shop I worked in was a Cadillac/Oldsmobile shop where we used AKSO-NOBEL/ Sikkens paint and primers. These are high-dollar, top-of-the-line products. Our production system was as follows:

    First, the bodyman finishes the filler work to 80-grit, then a quick trip over the filler and old paint edge with 180-grit on a D/A sander (lightly, to not destroy the shaping), then feather-edge the surrounding repair with 320-grit on a D/A sander, red scuff pad (Scotch-Brite) a little past the area to be primed. Prime with two to four coats of high-build, catalyzed urethane primer. After the primer is cured any areas needing slight block sanding are done with 240 to 320-grit on the primer. Then use a finishing D/A sander with 320-grit on flat surfaces and 320 to 400-grit dry sanding on most of the rest of the repair. For base-coat/clear-coat paint, the rest of the panel was wet-scuffed with an abrasive cleaner/sanding paste and a gray scuff pad (Scotch-Brite). Certain colors, mostly some types of metallics, would need a little finer prep on the primer area, maybe 500 to 600-grit light wet-sanding, and then clean-mask-sealer (often not necessary with this system), and then, paint.

    For those of you who are not familiar with a finishing D/A (dual action, orbital) sander, a regular D/A has an offset spindle that helps cut better for feather-edging and removing paint. A finishing D/A has less of an offset on the spindle, and with practice, produces a very smooth and flat surface without waves or ripples.

    My favorite suede finish is a very good quality single stage urethane enamel, flattened to about 75 to 80 percent. Some colors won't stand up with this much flattener in the mix, so some experience with the pigments used in the mix is a must. A big part of the durability in any paint system is in the primer undercoats.

    A friend of mine was restoring an old Corvette and the owner opted to buy a one-piece front end and then install the bonding strips to make it look original. I can't remember the company that made the front end, but it looked nice when it showed up. The installation went well and the paint looked real nice. This was in late winter early spring. The first time the car got some serious sunshine the seams and thicker areas of the glass started to bubble. He thought it was the paint, but when we inspected closer and I made a couple calls to my mom's cousin's husband (masters degree in industrial plastics) it confirmed what I suspected. The part was laid-up in January, when the shop was probably below 60 degrees Fahrenheit. They must have heated the outside to set it up, but the resin didn't reach full cure inside. After sitting in the sun, some of the curing process continued, which expelled styrene (a powerful solvent in polyester resin) and gasses, and caused bubbling in the areas mentioned. It was a mess.

    Fiberglass resin (polyester resin) is basically the same as what's in Bondo. In pure form it is like water, but usually the stuff we see is like syrup because it is thickened to make it easier to handle. The same basic ingredient (MEK peroxide) is used as a catalyst in Bondo and fiberglass resin. Mixing the correct ratio of liquid hardener to liquid resin is difficult and you need practice and experience. Temperature is extremely important. Below 64 degrees Fahrenheit the resin won't cure properly with the correct ratio of hardener. You can add a little extra hardener but too much and it cures unevenly and can trap gasses and styrene in the part or crystallize the resin and make it brittle. Over 95 degrees Fahrenheit it cures real fast and thicker areas can easily cure too hot and crystallize. It starts crystallizing over 140 degrees Fahrenheit. It's best if you can work between 75 and 85 degrees Fahrenheit (the temperature of the mold or part), and maintain some air flow and the correct hardener ratio, for a nice slow even cure.

    Different companies thicken their resin in different ratios, which proportionately affects the hardener ratio. It takes years of experience and knowledge to get real good at this, and occasionally the best glass gurus will screw up a batch.

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