188 Section 2 Nonstructural Repairs
and crack the filler. Cracked filler will allow moisture to
reach the metal, causing rust.
N O T E
A panel that has been previously damaged and repaired
may be damaged again. If the existing body filler is
thicker than 1/8", it should be ground out and the low
area should be raised. If the new fill area is adjacent
to existing filler that is 1/8" thick or less, the new filler
can be blended into the existing filler once the paint is
removed from the existing filler. See Figure 8-52.
The first step in filling is to closely examine the repair
area. The goal is to apply filler in a single application. There-
fore, high spots must be eliminated before filling. Look for
any high spots in the metal. Use a straightedge to locate
highs. If highs are found, tap them down with a pick hammer.
Next, remove the paint from the fill area and 1"
(25.4 mm) beyond the fill area. A grinder with a 24-grit
disk may be used to remove the paint on high-crown
steel panels. Grinding will leave deep scratches in the
metal. The filler will anchor in the scratches. This is called
mechanical adhesion. Because the heat produced by
grinding can cause panel warping, use only 80-grit sand-
paper on low-crown steel panels and all aluminum panels.
The scratches created by sanding with 80-grit sandpaper
will also anchor body filler by mechanical adhesion.
There are two acceptable surfaces that filler can be
applied over—bare metal and epoxy primer. For corrosion
protection, some manufacturers recommend that filler be
applied over epoxy primer. If epoxy primer is to be used,
remove the paint from the area to be filled. Mix the epoxy
primer and spray on the recommended number of coats.
After the epoxy primer has cured, it should be sanded with
320-grit sandpaper to provide scratches for mechanical
adhesion of the body filler.
Once the panel surface is prepared, mix the filler
according to the manufacturer’s recommendations. Read
the instructions on the can carefully. Mixing filler and
hardener is a critical step in panel repair. Using the wrong
amount of hardener will cause problems. The can of filler
will list the proper mixing ratio.
Always use clean mixing sticks, paddles, and mixing
boards. Dirty equipment can contaminate the newly mixed
filler. The mixing board must have a nonabsorbent surface,
such as metal or plastic. Never use cardboard as a
mixing board. One type of mixing board is a tablet of non-
absorbent paper sheets. After a sheet is used for mixing, it
is torn off the tablet, exposing the next clean sheet.
Before mixing filler, make sure it is uniform in consis-
tency. If a bluish liquid, which is the resin in the filler, is
visible inside the filler can, use a clean paint stick to mix
the resin back into the filler.
Use a clean paint stick or paddle to remove the
desired amount of filler from the can and place it on the
mixing board. Put the lid back on the filler can as soon
as possible. An open can of filler can be easily contami-
nated with the sanding dust present in the shop.
Paste-type hardener will separate inside the tube.
Knead the tube of hardener to thoroughly mix the sepa-
rated hardener. See Figure 8-53. Squeeze out the proper
amount of hardener next to the filler on the mixing board,
Figure 8-54. Use a clean paint stick or paddle to stir the
hardener and filler together. Continue to stir until the filler
is a uniform color, with no streaks of hardener or unmixed
filler. The filler is now ready to be applied in the low area.
The curing of the mixed filler produces heat, which
can be felt on the mixing board. The curing process
begins as soon as hardener and filler are mixed. The
mixed filler will stiffen as it cures and will only be spread-
able for a few minutes. Therefore, it is important to apply
Filler Primer
Figure 8-52. This panel contains body filler that was
applied during a previous repair.
Figure 8-53. Kneading the hardener tube will mix the
hardener.