198 Section 2 Nonstructural Repairs
Copyright Goodheart-Willcox Co., Inc.
■ The application site should be ground or sanded
and then cleaned to remove oil, wax, or any other
surface contamination.
The initial removal of excess filler is not necessarily
a sanding operation. When the filler has hardened to
the consistency of processed cheese, a surform tool,
commonly called a cheese grater, may be dragged
across the surface. The cheese grater works like a
file to remove excess filler. Grating is a fast way to
remove filler. It is faster than most sanding opera-
tions. However, do not grate the filler too soon. You
must allow the filler to cure long enough to adhere to
the metal. Grating too early will pull the filler off from
around the edges of the repair. The ragged edges
will require additional filler. If tack-free filler is used,
grating is not required. Allowing the filler to harden
completely and then sanding is an acceptable alterna-
tive. Figure 9-55 shows curing filler.
Fully cured filler is sanded starting with 40-grit
paper, followed by 80-grit paper and then 180-grit
paper. On small areas that the sanding tool can span,
start with 80-grit paper. Sanding before the filler is fully
cured will clog the sandpaper with sticky filler residue.
The filler can be tested for full cure by scratching it with
a fingernail. If the filler feels soft, moist, or sticky, it is
not ready for sanding. If the filler feels dry and hard,
it is ready for sanding. The sanding process will level
and featheredge the filler. Do not prime the filler for
at least ninety minutes after it is applied. Solvents will
evaporate from the filler even after it has cured. It takes
ninety minutes for all the solvents to completely evapo-
rate from the filler.
Block sanding can be done with hand or power
tools. In either case, the sandpaper surface should
span the filled area whenever possible. This allows the
undamaged edges of the panel to support the sand-
paper, preventing the paper from undercutting the filler.
If the filled area is too large for the sandpaper to span,
you must sand the filler as evenly as possible in sepa-
rate sections. The 40-grit or 80-grit sandpaper used to
sand body filler can easily scratch the paint of adjacent
panels. Protect the paint on undamaged areas with
masking tape and cardboard.
Block sanding levels filler. The goal is to apply one
coat of filler and block sand it level. Too many techni-
cians fill, block sand, find a high area, and tap it down.
They then refill this area, block sand, find a low area,
and refill again. Avoid wasteful refilling by identifying
all high and low areas, applying an overfill of filler, and
using proper block sanding techniques.
Block sanding can be one of the most difficult auto
body repair skills to learn. A common problem is over-
sanding the body filler, causing an undercut. An undercut
occurs when the filler has been sanded so much that it
is lower than the surrounding metal. The undercut must
be refilled and block sanded again. Sanding the filler to
match the surrounding contour, such as double crown or
a body line, is another potential problem.
First, consider a 6” (15 mm) diameter section of
body filler. A sanding stick can span this amount of
filler. Hand placement on the sanding stick is impor-
tant. Hands should be placed at either end of the stick
and they should be supported by the undamaged metal
around the filler. The undamaged metal will serve as a
guide for block sanding and will prevent undercutting.
Always follow the crown on the panel when sanding.
Block sanding low, medium, high, and double crown
is shown in Figure 9-56. Hold the sanding stick flat
on low or medium crown. See Figure 9-56A. On high
crown, hold the sanding stick on edge as shown in
Figure 9-56B. If the sanding stick is held flat, the
high crown will be sanded flat. When sanding double
crown, the stick must be bent to follow one of the
crowns, Figure 9-56C. On small areas, start sanding
with 80-grit sandpaper. When a featheredge appears
around the filler, switch to 180-grit sandpaper to
remove the 80-grit scratches and to extend the feath-
eredge. If there are no problems with the filler, such
as pinholes, the filler is ready for the undercoat. Some
body shops apply a thin coat of premium body filler
over the filler and sand that filler with 180-grit sand-
paper followed with 320-grit sandpaper.
If pinholes or other low areas are present, they
should be featheredged and filled with body filler.
Figure 9-57A shows an edge that could not be feath-
eredged because the area is still low. This area, as
well as the low area that remains after block sanding,
should be filled as shown in Figure 9-57B. Once the
second application of filler has cured, sand it with
180-grit sandpaper. Sand on the new filler only, not the
surrounding filler.
Goodheart-Willcox Publisher
Figure 9-55. Allow the filler to cure before sanding.
Cured filler