Painting 101: Understanding And Properly Using Body Fillers

Automotive body fillers seem simple enough on the surface. Buy a can of product, mix it up, spread it on, let it harden, and then sand it. You don’t even need expensive tools to use it. But, what happens when it doesn’t harden? Or a year or so later, you notice a crack along the edge of that repair on the bottom of the quarter-panel? Body fillers are one of those products you don’t think about until there’s a problem.

The best way to avoid those problems is to use the right product for the right task. It’s also not the place to use the least costly alternative. Contrary to common opinions, all body fillers are not created equal.

body fillers

Here’s a quick way to get a correct ratio of product. Pour out a four-inch puddle of filler and squeeze a strip of hardener across it. Most products are mixed at a 50:1 ratio, but always check the instructions for the product being used. Evercoat Quantum1 also comes in a handy cartridge dispenser and takes the guesswork out of mixing ratios. This reduces the risk of problems due to inconsistent mix ratios. Quantum1 is a unique product that combines the adhesion and stability of an epoxy with the sandability of a polyester.

A filler that works best for shallow dents might not be suitable for significant panel straightening. If you need to rebuild a corner area, a high-solids, super-strong filler will be needed. When doing bodywork on a seam – like a roof/sail panel seam – a metal or carbon-fiber-infused filler is needed. And, what exactly is putty? Is it different than regular filler, and when should it be used?

What Makes A Good Body-Filler

We rate fillers on two factors – durability and workability. Durability refers to how well the product will hold up over time. Metal parts and panels can vibrate and flex. Over the years, areas repaired with filler might lift or crack. Filler staining is also something to consider. Staining is caused by a chemical reaction between the filler’s resin or hardener and the paint. This can be minimized by using a quality filler.

Workability means how easy the product is to work with. Does it sand easy? When its sanded, does the removed material have the consistency of powder or is it more coarse or gritty? This is referred to as powdering. Then there’s pinholes. Using a proper mixing technique will reduce the pinholes (air pockets) that form in the product. Plus, the better the quality of the product, the less tendency there will be for pinholes to form.

Standard-Grade Body Fillers

These are a lightweight product that spreads easy and cures fast. They are used for repairing minor dents and scratches. Standard-grade fillers have minimal adhesion properties and offer the least stain-resistance. While the price makes it tempting to purchase this for a restoration project, these fillers are on the low end of the durability and workability spectrum.

body fillers

Filler technology is always improving. Evercoat’s Rage Optex is a premium filler. It’s easy to know when the product is fully catalyzed and ready to sand. When it turns green you can start sanding.

Medium-Grade Body Fillers

These lightweight fillers are designed for applications where high-productivity needs to be balanced with premium results. They are used in body shops for small repairs. They sand more easily than standard fillers, adhere well to most surfaces, and have better stain-resistance. Evercoat Products makes two medium-grade fillers, EverGold and Z-Grip. EverGold levels to a smooth finish without leaving a bumpy texture, and sands easily with 80-grit sandpaper. Z-Grip is a metal-infused filler that is easy to sand, and provides superior adhesion.

Premium-Grade Body Fillers

These are made from high-quality resins, and are the best product to use when doing intensive body resurfacing – making multiple filler applications and sanding. They have the highest level of stain-resistance, and sand the easiest. High-quality fillers will also level out flat, cure quickly, and are the easiest to work with. They powder nice and tend not to clog up the sandpaper. When you’re trying to get the side of a car flat and straighten up the body lines, this is what you need to be using. These are at the high-end of the durability and workability spectrum, because they’re designed to last long and sand easy.

Single-Step Fillers

These are a premium filler. Products like Evercoat’s Quantum1 are a combination of polyester and epoxy. They have less shrinkage, sand easily, and better repair holdout than any other fillers. The product sands as well in 30 minutes as it does in 30 days. It sands two times faster than traditional fillers and requires less sandpaper. Because it sands so easily, you can use a less-coarse sandpaper and not deal with deep sand scratches. Some of these fillers can be applied over silicon-bronze welding without adhesion issues.

body fillers

Single-step fillers like U-Pol’s Dolphin One Fill are not cheap. But, they could be worth it if you’re restoring your valuable automotive investment.

Now for the big question, how much of a difference is there between the standard and premium? Many folks will say ‘so it sands a little easier, is that worth paying the extra bucks?’ For that, we asked Vinny Liotta of U-pol products. “Far and away the best filler you can buy for your project is the best option you can use. The higher-end fillers, like our Dolphin One Fill, have higher-grade intelligent microsphere technology. That means they’re solvent impervious; solvents do not attack and swell them, which can create problems later. They are also easier to sand, both sooner and later on. This high-end filler can sit for long periods and still sand like it was applied yesterday.”

Reinforced Body Fillers

These are body fillers that contain strands of either fiberglass, Kelvar, or carbon fiber. They are used for major repairs when filling holes, or fixing shattered or torn fiberglass panels. The fiberglass products are divided into short-strand and long-strand. Long-strand is used for major fiberglass repair in large areas and is excellent for bulk filling. Short-strand is used for less intensive repairs. It’s excellent for use on panel-bonded seams, weld areas, door edges, and areas prone to water exposure. Using fiberglass fillers gives extra strength to repaired areas. Some technicians will apply a layer of short-strand fiber to an area, and then apply a layer of premium filler to smooth the surface. These products dry very hard and are harder to sand than regular fillers. They should also be skim coated with a traditional filler.

Carbon-Fiber Filler

These are high-adhesion fillers reinforced with carbon-fiber strands to give structural strength to a repair. They are great for use on edges, corners, and over welded seams where lead was originally used.

body fillers

Carbon-fiber reinforced filler is a favorite among restoration professionals. It’s very strong, which make is perfect for use along edge areas to fine-tune panel gaps after welding. U-Pol SMC is one of the few fillers made with carbon fiber.

Metal Fillers

While some professionals will use lead and soldering tools, many will use filler to seal welded seams. Technology and new products have made it much easier to get great results without using lead. One of those products is metal reinforced filler. These products also have great adhesion to steel, galvanized steel, and aluminum.


Putties and glazes are smoother and creamier to apply than filler. They are designed to be applied in thin coats to fill pinholes, scratches, and other small imperfections. However, companies like Evercoat have created new-technology plastic fillers that are eliminating the need for putty. Evercoat explained it like this. “The difference between putty and filler is basically the “talc” in the product. Most of our Rage line of fillers have Eco-Resin technology. With that, we use similar ingredients to putty. Therefore, when using it, the putty is no longer necessary in the repair process.”

body filelrs

Putty is great for filling pinholes, scratches, and minor flaws. Metal Glaze is a two-part, premium polyester finishing and glazing putty. It has great adhesion and is self-leveling. Reinforced fillers have many uses. Fiber Tech filler is reinforced with short- and long-strand Kevlar. It’s a very versatile product that can be used for filling holes, repairing cracks, or any kind of gap filling. Apply it over metal or any rigid composite material.

Getting The Most Out Of Body Filler

  • The most common mistake when using filler is under catalyzation. Folks looking for longer work times, don’t add enough hardener and think it will sand easier. This is the single-worst mistake to make, because the filler never gets hard enough. Any soft material must be removed, and the filler application process is started again. Always combine the exact amounts of filler and hardener as instructed on the packaging.
  • Never apply filler over rust. The filler may seem like it’s sticking, but eventually, it will pull away from the rusted surface. Remove the rust, then apply the filler.
  • Don’t use cardboard to mix filler. The absorbent properties of the filler can be affected by the cardboard. Use a hard-surface mixing board or a mixing board with tear-away sheets. The sheets are non-absorbing, and one pad has 100 sheets.

Some fillers, like Evercoat’s Metal-2-Metal, use a liquid hardener. Here, the products are being mixed on a tear-away sheet of a disposable mixing board. Never use cardboard to mix fillers.

  • Give the filler a good rough surface to hold onto. Don’t apply filler over unsanded metal. Make sure the filler has a good deep tooth to grip. For metal, that means sand or media blasting. For all other surfaces, make sure to use no grit higher than 220. Most of the work we do is done with either 80- or 120-grit.

  • Work on plastic-filler projects on warm, dry days, rather than humid days. It will help the filler to cure faster.
  • Be sure to wear a dust mask when sanding – breathing sanding dust is as bad or even worse than breathing paint!
  • Don’t wander away after applying the filler. You might want to “rough in” the surface. There will come a point when the filler is hard enough to carve or sand, but soft enough to shape easily. Wipe over it with a piece of 80-grit. If the sanding debris falls loose and doesn’t clog the paper, sand and shape the surface. Then allow it to fully cure and do the finish sanding.
  • If using putty or glazing products, be sure to allow the product to fully cure before sanding. If the product doesn’t ‘powder’ when sanded, give it more time to cure.

Fillers are rated by how well they powder when sanded. The material that sands off should have the consistency of powder. It’s also a great way to know if the filler is fully hardened, as only cured filler will look like powder when sanded.

  • For minimal pinholes – don’t stir the filler and hardener when mixing. Fold the hardener into the filler, squeezing out the air pockets. Apply the product in a smooth, even coat.
  • If applying filler over a silicon-bronze TIG-weld, apply a coat of epoxy primer before spreading filler. Or, use a filler that is designed to apply directly over this kind of weld. Evercoat’s Quantum1, Rage Ultra, and Rage Optix are all recommended with no primer.
  • Be sure to thoroughly mix the filler before taking it out of the container. There’s usually a layer of resin on the bottom of the can. If the filler comes in a pouch, knead the pouch for a few minutes before squeezing out the product. And always knead hardener tubes before using.
  • Most products have a tech sheet available on the manufacturer’s website. Download the tech sheet and read through it before using the product. It will explain exactly how the product should be used, and answer most questions that will come up.

Always grind your welds before doing bodywork .

Plastic Spreaders VS. Metal Spreaders

This is a hotly debated subject. Many technicians don’t like metal spreaders, while others swear by them. It’s a matter of personal preference. One trick is to buy a whole box of plastic spreaders and use a fresh one each time. A new, fresh spreader surface will apply the filler more evenly.

This wraps up the first installment of our Painting 101 Series. Our next edition will feature everything you always wanted to know about primers. If you have any questions or suggestions, please don’t hesitate to reach out. We want this series to be as helpful as possible!

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About the author

JoAnn Bortles

JoAnn Bortles is an award winning custom automotive painter, airbrush artist, certified welder/fabricator, author, and photo journalist with over 30 years of experience in the automotive industry.
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