But if you understand the basic composition of plastic fillers,
how they work and how to properly apply them, you’ll have a good
grip on how to use them in your shop.

Filler Basics

The four basic categories of plastic filler
are heavyweight or conventional fillers, lightweight fillers,
premium fillers or basecoat/clearcoat fillers, and finishing fillers
and/or putties (there are also fiberglass-reinforced fillers and
metal fillers, but we’ll save them for another time).

Plastic fillers are basically a mixture of
polyester resin, talc and/or microspheres. The premium and lightweight
fillers will have microspheres, as well as talc, mixed in the
polyester resin. Conventional or heavyweight fillers are basically
polyester resin and talc. Finishing fillers and putties – designed
to spread on thin to cover small imperfections in the metal work,
to sand easily and to adhere to most any surface – have their
own special formulation.

Talc is a soft mineral consisting of a basic
magnesium silicate. It looks like baking soda and is much heavier
than the newer microspheres, which look like miniature Ping-Pong
balls (this is an overly simplified description, but it gives
you an idea of what these two ingredients look like before they’re
mixed with the polyester resin).

It’s interesting to note that there are different
sizes of microspheres and different grades – or finer grinds –
of talc, and each manufacturer guards the exact ingredients and
percentages of talc, microspheres, etc. One thing to keep in mind:
As the percentage of microspheres increases, the filler becomes
lighter, is less tacky or sticky when it cures and produces less
dust when sanded.

Also keep in mind that talc isn’t a waterproof
material. This is important when performing a rust repair. If
the rust out isn’t completely sealed against moisture, the filler
can – and usually does – draw moisture, which means the repair
will fail.

Premium Fillers

Take the time to read the product label of
a premium filler. Does this sound familiar: "Brand ‘X’ is
a quality, homogenized premium filler that is tack free in just
15 to 20 minutes; features less sandpaper clogging and excellent
featheredging; has excellent sanding properties; has excellent
adhesion to properly prepared galvanized steel, ‘E’ coats and
SMC; spreads smoothly; doesn’t sag on vertical surfaces; virtually
eliminates pinholing; and is guaranteed to be stain free."

All of this marketing information is true
and accurate until the technician uses it under less than ideal
conditions or without heeding the specific product instructions.

It’s important to remember that premium, stain-free
fillers; finishing fillers; and putties will have a higher cost
but better-quality ingredients. The purpose of better-quality
ingredients is to help eliminate the potential for staining. For
fillers, this means a higher-grade polyester resin is used.

Premium, stain-free, tack-free plastic fillers
are more expensive than conventional or even lightweight fillers,
but the advantage they offer is the reduced risk of a redo because
of staining. This is the plain, simple truth.

Another point to remember is that quality
metal repairs will reduce the amount of filler needed to restore
the shape and contour of a damaged panel. Poor metal work automatically
and needlessly increases the consumption of expensive products.

Stain-Free Repairs

In the middle of each plastic-filler manufacturer’s
product line are the lightweight fillers, followed by the conventional
or heavyweight fillers. These products aren’t guaranteed to be
stain free, but because they are typically lower in cost, some
shops have elected to use them.

What the technician will try to do is completely
cover the conventional or lightweight filler with one of the stain-free
finishing fillers or putties. If the stain-free product completely
covers the filler and is the right thickness after final sanding,
stain-free repairs are possible. Most high-production shops think
this is a matter of stepping over dollars to pick up a few pennies.
You have to decide if this procedure is right for your shop.

Note: Most refinishing today automatically
means the use of basecoat/clearcoat paint systems, and the cost
of redoing just one paint job could easily have paid for a higher-priced
premium filler.

There are two types of staining: bleaching
and bleed through. Bleaching occurs when the filler is over-catalyzed
with hardener (benzoyl peroxide). The extra hardener can be drawn
out by the aggressive solvents found in today’s finishes, and
when the extra hardener activates with the pigments in the colorcoat,
bleaching occurs. To avoid bleaching, make sure you read the manufacturer’s
instructions carefully and use the correct ratio of hardener to
filler.

The second type of staining, bleed through,
occurs when the aggressive solvents in the clearcoat pull out
the photo-reactive chemicals created by the activator and hardener.
This reaction is a normal process when you catalyze plastic filler.
It may take weeks or even months, but this type of staining will
intensify when exposed to sunlight and ultraviolet rays. Light
colors, particularly white, seem to be the most likely candidates
for bleeding.

How do you eliminate staining? The first step
in eliminating bleaching and bleed through is to use a premium,
stain-free filler. The second step is to use the right amount
of hardener, and the third step is to use a stain-free finishing
filler or putty, if necessary. If you want an almost 100 percent,
iron-clad guarantee against staining, you also should use a two-part
primer surfacer/sealer and follow the paint manufacturer’s recommendations
in applying it.

If those precautions haven’t been taken, it’s
possible you’ll see a stained panel in your shop. What’s the correct
procedure for refinishing it? The recommendations may vary, but
one option is to start over by removing the topcoats and filler
to bare substrate. Use stain-free products and then prime with
a two-part primer surfacer. Another option: Simply sand the clearcoat
and reapply the basecoat/clearcoat finishes. Some discussion with
your jobber and the paint manufacturer could prove helpful in
such an instance. The last thing you want to do is repaint a panel
for a third time.

Application Tips

As with any other shop product, there are
common trouble spots associated with plastic filler. Shop temperature,
application technique and contaminated surfaces are just a few
causes for this substance to perform less than perfect.

Keep the following trouble spots and application
tips in mind the next time plastic filler is required for a repair
in your shop:

• A cold shop can cause slow curing time, allowing the filler
  to remain too soft and causing excessive shrinking. How do you
  avoid this? Fillers are designed to work between 60 and 90 degrees
  F, so if you need to warm a panel, make sure you don’t exceed
  this recommended maximum temperature. You can speed up the filler
  curing time by heating to about 120 degrees F, but be careful.
  Too much heat applied too close can actually blister the filler,
  so you have to pay attention to what you’re doing.

• Surface prep is very important in obtaining good plastic-filler
  adhesion. More and more technicians are catching on to the fact
  that high-speed power tools with a 36-grit, fibre-backed, course
  sanding disk are too aggressive. Slow-speed power tools with a
  heavyweight sandpaper remove less metal and create less heat on
  the newer, thin-gauge metal panels.
• Sanding plastic filler has changed as well. The new premium
  fillers can easily be sanded with an 80-grit sandpaper. With these
  fillers, cheese graters and 36-grit sandpaper are no longer needed.
  If you’re working with conventional fillers, however, then there’s
  still some benefit to using a cheese grater and course-grit sandpaper.

• It’s no secret that plastic filler won’t adhere to a dirty
  or contaminated surface. Begin every repair by cleaning with soap
  and water, followed by a wax and grease remover. This is a fundamental
  step that’s often neglected to get the job done faster.

Once the repair has started, contamination could come in the form
of oil from an air tool. What do you do now? The smart technician
will take the time to clean off the bare steel with a wax and
grease remover, or even acetone, and will then give these solvents
time to evaporate completely before applying the filler.

• Most of the new premium fillers are designed to adhere to
  galvanized steel and aluminum, but be sure you check the product
  label to verify this fact. If the filler and hardener are mixed
  correctly and allowed to cure properly, adhesion shouldn’t be
  a problem; adhesion problems occur when technicians rush through
  jobs. Some manufacturers will recommend light sanding of galvanized
  steel or the "E" coat to increase adhesion, so pay attention
  to the instructions for the specific filler being used.

Through the years, some technicians have applied one coat of a
two-part epoxy primer to both galvanized steel and aluminum prior
to applying the plastic filler. This method not only provides
a "grip" for the filler, but also enhances corrosion
protection in the repair area. Some techs have been successful
with this method on rust repairs as well. However, this method
may not be approved by all filler manufacturers, so be sure you
test the procedure or call the filler manufacturer first.

• Pinholes can be another troublesome problem. One of the most
  common causes of pinholes is improper mixing techniques. Filler
  and hardener should be folded during mixing: fold it, smash it,
  spread it, fold it, smash it, spread it … you get the idea.
  Stirring causes air bubbles that cause pinholes. Applying filler
  too thick in one application can also cause pinholes. Your technique
  in applying the filler can also be the cause – firm pressure is
  needed. Other causes of pinholing include not allowing a cleaning
  solvent to fully evaporate prior to applying the filler, too much
  hardener and high humidity.

Product Knowledge

When using plastic fillers, there’s no substitute for developing
a system and selecting a single product line. As with any product,
you must also read, understand and follow the product label.

Always check new containers of filler and hardener to make sure
they haven’t separated or become lumpy. Also, knead the hardener
before using it. If you can’t establish a uniform color or consistency,
throw it out – this goes for both the filler and hardener. The
product may be out of date or it may have been exposed to too
much heat, stored wrong or defective at production (a rare occurrence,
but a possibility).

As with any repair process, you need more than a general knowledge
to perform quality work. Before you jump right in and start working
with plastic fillers, get filled in on the details first.

Writer Fred Kjeld is a contributing editor to BodyShop Business.

Future Fillers

Maintaining the vehicle manufacturer’s anticorrosion warranty
is often forgotten in the rush to finish the repair. But what
if you had a filler that – in addition to adhering to cleaned,
ungrounded, galvanized steel or a steel panel with the topcoat
removed – would retain corrosion protection from the OEM’s "E"
coat? What if there were a filler that needed no hardener?

New products are designed to simplify the repair process, improve
corrosion protection and gain an economic advantage. When trying
new products, read the product information carefully and test
the product in your shop under your everyday conditions using
pieces of scrap panels that you’d normally throw out.