190 Section 2 Nonstructural Repairs
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 sandpaper, 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 separate 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- r r
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 important. 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 8-56.
Hold the sanding stick flat on low or medium crown. See
Figure 8-56A. On high crown, hold the sanding stick
on edge as shown in Figure 8-56B. 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 8-56C. 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 sandpaper followed
with 320-grit sandpaper.
If pinholes or other low areas are present, they should
be featheredged and filled with body filler. Figure 8-57A
shows an edge that could not be featheredged 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 8-57B. Once the second application of filler has
cured, sand it with 180-grit sandpaper. Sand on the new
filler only, not the surrounding filler.
Areas larger than 6" (15 mm) in diameter can be initially
sanded with 40-grit sandpaper on an air file as shown in
Figure 8-58. The air file will level the filler much faster than
hand sanding. Try to sand the entire area gradually and
evenly. Sand until a featheredge is made all around the filler.
Then switch to 80-grit sandpaper. At this point, use hand
sanding rather than power sanding because hand sanding
is easier to control. Large areas of filler can be sanded with
a yardstick instead of a sanding stick. Because the yardstick
is longer, it can span a wider area of filler. It is also flex-x x
ible, so it can be bent to conform to a medium crown. After
removing the 40-grit scratches with 80-grit paper, switch to
180-grit sandpaper to remove the 80-grit scratches. To sand
a crowned area around a wheel opening, use the curved
surface of the sanding block as shown in Figure 8-59.
Sanding a body line in filler is challenging. It can be
difficult to sand a straight line that matches the undam-
aged contour of the body line. Any deviation will be easily
visible in the repaired vehicle. One way to form a body
line out of body filler is to use a body line sander, such as
the one shown in Figure 8-60.
A conformable sander is another tool that can
shape body filler. The conformable sander is pressed
into an undamaged portion of the body line. The thin
plastic sheets take on the contour of the body line. The
sandpaper must be carefully applied to the contour of
the sander. A roller tool is used to make sure that the
sandpaper matches the contour of the sander exactly.
The guide is positioned so the sander can cut the body
line in the filler. It is held to the vehicle with suction cups.
N O T E
The guide’s suction cups will adhere only to painted
surfaces.
Figure 8-55. A surform tool can be used to quickly
remove excess filler.
Surform tool