BodyShop Business
Fill In The Blanks: Correctly Using Fillers
Mark Clark
8/1/2000

As sheet-metal panels become thinner, fewer panels get repaired and more panels are replaced. In fact, one statistic suggests that as many as 75 percent of autobody panels are replaced in today’s efficient collision repair shops. If only 25 percent of panels are repaired — with a dent or crease removed — then less body filler is being used these days.

Mostly gone are the days when the sleazy used-car guy down the street used entire 5-gallon pails of filler weekly to avoid doing much metal work. Consumers demanded better. Today’s fillers are used in smaller volume, but come in a wider variety of styles — providing just the right product for each application during repair.

So Many Fillers, So Little Time ...
Poly is a Greek word that means "many," so polyester describes a chain of many chemical "esters." By that same reasoning, polyurethane describes paint made from many different resins. By mixing polyester resin and a fill material, body filler is created. What the fill material is made from differentiates types of body filler.

  • Talc — The original body fillers that replaced lead are filled with talc. Talc is hard clay that often has a fancy name like "magnesium silicate" but is really just mined out of the earth. Filler with talc isn’t waterproof because the talc wicks the moisture right through the repair. You know this is true because every time it rains, the water on your lawn is absorbed into the earth by the clay in the soil. If talc were waterproof, your front yard would be under water every time it rained.

  • Aluminum or fiberglass — Moisture-resistant fillers use aluminum or fiberglass as fill material. Neither will absorb moisture like talc does.

Mixing long strands of fiberglass into the polyester resin makes waterproof filler that’s also very strong. Why? Because the strands of fiberglass cross each other at every possible angle. However, it’s difficult to sand the very hairy filler to a smooth surface.

Using chopped-up fiberglass as fill makes for a non-talc filler that’s much easier to work with than the long stuff and is still waterproof.

Finely ground aluminum body filler is not only waterproof, but also spreads easily and gets so hard you can drill and tap it. So why doesn’t everyone use aluminum-filled body filler? Have you priced a can lately?

  • Plastic — Then there’s lightweight body fillers, which use microspheres as filler. Unlike pieces of talc — which are not only surrounded by the polyester resin but also absorb it — microspheres are surrounded by resin but don’t absorb any. As their name indicates, microspheres are tiny globes of plastic. Available in several sizes, they reduce the weight of the filler and speed the cure time.

Since they’re hollow plastic and won’t absorb any resin, they weigh less than a resin-soaked chunk of clay. And since they’re only surrounded by resin, there’s less resin mass for the hardener to catalyze so lightweight fillers harden faster. Because most lightweights also contain some talc, they’re not waterproof.

The Role of Hardeners
The hardener for polyester resin has an oxidizing agent that creates heat and cures or hardens the esters into a rigid form. Paste hardener has benzoyl peroxide as its agent, and liquid hardener has MEK peroxide to create the necessary heat. Typically, paste hardener is about 49 percent benzoyl peroxide, 49 percent plastic carrier agent and 1 or 2 percent colored dye. The dye is present to ensure a thorough mix with the body filler.

A uniform color indicates even mixing. Many premium fillers come with specially colored plastic spreaders. The goal is to mix the paste hardener in an amount that yields body filler tinted the exact same color as the plastic spreader.

Determining the correct amount of hardener will vary with the temperature in your shop. Hot weather and hot metal surfaces will make the filler cure faster. Conversely, cold temperatures and cold metal surfaces will make the filler cure very slowly. Many body filler directions call for adding a 1-inch ribbon of hardener to a golf-ball-size lump of filler. But most technicians go way too fast to calculate how many golf balls would fit in their puddle of filler. One common piece of folk wisdom is to rap the mixing board on the edge of the workbench until the body filler flows down from a mound into a puddle. Then lightly squeeze the tube of hardener to make a letter "X" across the puddle. Hardly a precise measurement. The real test is the color of the mixed filler and the cure time. If the filler doesn’t have a noticeable color difference after mixing, not enough hardener was used. If the filler is a dark, vivid shade, it’ll likely set up way too fast because too much hardener was used.

The tube of paste hardener has a relatively short shelf life. All body filler comes with some sort of date coding (i.e. "Best if used by December 2000"). So, if you find a case of filler hidden on the bottom of your shelf with a date long past, you may only need new tubes of hardener to make it work well.

In a heated storage room, the polyester body filler will last several years. It’s just the hardener that goes bad. Paste hardener should be kneaded in the tube to reach a smooth uniform consistency every time before it’s used. If the paste feels chunky or watery, looks milky or clear and won’t go back together, you need new hardener.

Tips On Body Fillers

  • When the lid comes off a new gallon can of filler, you’re likely to see a layer of liquid resin floating on the top. What’s happened is the fill material (talc, aluminum or fiberglass) has sunk to the bottom to the can. Just use a sturdy mixing stick and slowly stir the resin back into the filler.

When you open an old can of some lightweight fillers, you may see a dry crusty layer on the very top. What happened is the very light microspheres floated to the top of the resin, leaving a dry looking product. All that’s required is the same careful top-to-bottom stirring action. If the filler won’t re-homogenize with a reasonable amount of stirring, discard the can.

Body filler is designed to be used in temperatures ranging from 60-90 degrees F. Within that range, the filler should be ready to cheese grate in 10 to 12 minutes, ready to sand in 20 to 25 minutes and completely cured in 60 minutes.

  • Body filler should be applied in thin layers from 1/8-inch to 1/4-inch thick, with dry time between layers.
  • Polyester resin body fillers dry naturally with a sticky or tacky top surface. The catalytic heat is trapped in the layer of filler by the surrounding metal dent and by the filler itself — except at the very top where the heat can escape, preventing complete cure and leaving a tacky surface. You can simply sand off the very thin layer of sticky resin with an 80-grit disc or use tack-free filler. Tack-free body filler contains paraffin wax, which floats to the top surface of the repair and prevents heat from escaping. You can do the same thing by covering the filler with wax paper as it cures or even get filler to dry tack-free by applying external heat from a quartz heat lamp. Beware of washing off the tacky resin with strong solvents. The solvent is wicked into the filler by the talc and will cause the filler to remain soft.
  • Too much hardener or improper mixing and spreading can cause pinholes. The pinholes may simply be air bubbles that were whipped into the filler when stirring the new can. Never put body filler on a paint shaker because violent agitation can cause air bubbles. When adding the hardener, don’t stir the mixture. Instead, fold the filler over on itself and push the hardener into the resin.

High humidity can cause the filler to trap moisture, which can cause pinholes, as well. And applying the filler too thick can cause gassing and air entrapment, which will also show up as pinholes.

  • Adhesion of body filler is dependent on good surface preparation. If the area isn’t clean and free of wax, grease or other contaminants, the filler won’t stick. Getting filler to stick to galvanized metal can be difficult. Keeping the filler from rolling back at the thinnest tapered edge can also be a challenge. Special additives or adhesion promoters cause the resin to adhere better to the zinc coating and are well-worth their extra cost. It’s much better to buy special fillers designed to stick to galvanized than it is to grind off all the corrosion-resistant zinc.
  • To get the best adhesion, push the first coat of mixed filler tightly onto the clean, sanded substrate in thin coats. Grind the repair area with 24-, 36- or 80-grit sandpaper first.
  • All fillers tend to be weak at their thinnest point. On the very tapered edge of the repair, there’s very little filler mass and, as a result, not much heat. Without the heat, the filler doesn’t cure thoroughly and peels off when sanded. External heat from a quartz light will help the thin, outer edges stick better.
  • Using too little hardener can cause the same rollback problem on the thin edge. Use the right amount for the changing conditions in your shop.
  • One common problem is featheredge sanding too soon. Wait 20-25 minutes before sanding for maximum adhesion.
  • Another whole set of problems –– including staining, poor cure, sand scratches and poor topcoat adhesion — can plague body filler when it’s painted over, and all of them can be traced to the solvent in the primer, the sealer, the color and the clear. Because most fillers contain some talc, they’ll readily absorb solvent and hold onto it. Since new high-tech clearcoats are made from very sophisticated resin technology, they require strong solvents to dissolve them. This is why you seldom see a filler problem appear under the primer or even the color coats. It’s the clear that has the strongest solvent base and can cause the most damage when sucked into the filler. Waiting an hour for the filler to cure before painting anything over it is a great way to prevent problems. You can’t elect to use milder solvents because they won’t dissolve and flow the snazzy clearcoat.

Is That a Stain I See?
Staining in body filler is less common than you might think, based on the popularity of stain-free fillers. I often ask painters in my audiences if they’ve ever had a filler stain, and many say no. Many others have gone years between stains.

Most often seen on light colored paints, the stain can appear as an amber yellow or off-white patch under the clear. What’s happened is that strong solvent has been absorbed into the filler.

What actually causes the stain is open to interpretation. One hypothesis is that the peroxide agents in the hardener are pulled to the surface by the escaping solvent. Once there, they can bleach the pigment out of some colors or even react with some pigments to cause a visible stain.

If you’ve dramatically over-catalyzed the filler, the stain may also have a dark ring around the edge of the repair. If you’ve applied the filler too thickly and not allowed dry time, the absorbed solvent can actually release a pocket of "gas" that can alter the pigmentation and cause a stain.

To minimize the chance of a stain, wait the recommended dry times before applying anything over the filler. Be careful not to force dry a repair too soon either. The external heat from the drying light or the oven can drive the solvent deeper into the filler.

To prevent staining, keep the solvent from touching the talc containing filler. Use a two-part primer over the filler as a barrier coat. Either polyester, epoxy or urethane primer surfacers will act as a firewall to turn back the encroaching solvent. Two-component sealers add even more protection from solvent absorption into the filler.

Like many who have been around the industry for a long time, I first saw a filler stain on a white pearl custom paint job. After months of careful work, the painter was devastated to see a huge yellow stain on the repaired quarter panel — which didn’t appear until the car was parked outside under the hot sun for several days. In those days, the custom finish of choice was acrylic lacquer. This car had multiple coats of acrylic lacquer primer surfacer, lacquer sealer, white acrylic lacquer color, white pearl midcoat and many coats of acrylic lacquer clear. The color, the pearl and the clear were all thinned with the slowest, best flowing lacquer thinner available.

Many of you remember just how potent PPG DTL105, DuPont 3602S and R-M PNT90 are. They were absorbed deeply into the conventional style, talc-filled body filler. When the hot sun had a chance to heat the repair, the thinner evaporated and carried some free agent along with it that bleached the white color yellow.

Today, a nice 2K primer or sealer would likely have kept the lacquer thinner from ever reaching the body filler.

Stain, Stain Go Away ...
Once a stain appears, how will you repair it? One possibility is to remove all the topcoats and filler and start over, making sure to apply the filler in thin coats and allow plenty of dry time before painting. Leave at least 5 mils of 2K primer and/or sealer between the filler and the color coat to act as a barrier.

Don’t want to take it all off? I don’t blame you. Another possible repair is to sand the stain off below the filler surface without removing the bulk of the filler. Apply 2K spot putty, allow plenty of dry time, and re-apply the color and clear.

An easier repair is to just sand the color off down into the primer surfacer, re-apply the 2K primer, re-color and re-clear.

Fastest of all is to simply sand off the clear and shoot more basecoat directly over the yellow stain. Fully cured topcoats make for a sturdy barrier coat themselves. In my experience, this last version has worked very well. I’ve never had filler re-stain in the same spot, but it’s possible. The surest solution is to remove it all and start again, but I’d certainly try to just re-color and re-clear first.

You can probably avoid the problem entirely if you use the recommended amount of hardener, wait the appropriate dry times and apply a 2K-barrier coat over the filler.

Filler Up!
Today’s specialty fillers offer tack-free, stain-free, galvanized adhesion and easy sanding. If your shop is buying the cheapest filler possible to save money, you’ll likely find that a more expensive filler with better resin, better talc, microspheres and selective additives will do a better job, and do it faster.

Labor time remains the most expensive thing in any body shop. At $34 per hour, a 10-minute savings per repair adds up to $5.70 — well-worth the extra cost for the good stuff.

Writer Mark Clark, owner of Professional PBE Systems in Waterloo, Iowa, is a well-known industry speaker and consultant. He’s been a contributing editor to BodyShop Business since 1988.

Out with the Lead, In with the New

Body filler was invented to replace lead — which is very difficult to use — to restore bent sheet metal to contour. Many years ago, I had several lessons from older body technicians on spreading lead. I never got the hang of it. The problem occurs on vertical panels ... that darn gravity makes the job tough.

Body lead is also correctly called bar solder. Since it’s solder, it turns molten at fairly low temperatures. The trick is to get the surface of the metal to the same temperature as the stick of lead (after you’ve treated the bare metal to accept the solder). By passing a soft flame from the acetylene torch equally over the panel and the stick, you can push the molten lead onto the panel. Once it’s puddled on the panel, you smooth it out by pressing a wooden paddle coated with beeswax over the warm surface. I was actually able to do this successfully once or twice. My problem occurred after I started filing the patch with a metal file and discovered I needed more lead. Guess what? As soon as I got the metal temperature as warm as the stick of lead, the existing puddle turned molten too (duh!) and ran down the side of the car. No wonder polyester body filler is so popular. It’s easy to use.


A Note About Spot Putties

Spot putties that use paste hardener are nothing more than finely ground body fillers. By running the talc through a grinder more times, a finer, smoother putty is possible. Adding smaller microspheres to lightweight putty also makes for smoother spreading. Some shops insist on using tubes of lacquer spot putty, claiming that the time to mix 2K putties is inconvenient. The problem is the tendency of the lacquer resin to absorb solvent from subsequent color and clearcoats. It swells and shrinks as solvent is absorbed and released. Catalyzed spot putties won’t swell as solvent is applied and will stay flat once sanded flat.


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