If you are working with larger
Old body shop practice was to use panels, like whole bodies, hoods,
36-grit, and even extremely coarse
decklids, or doors, you should con-
24-grit, abrasive papers for this pur-
sider dip stripping as a method of
pose. Open-coat abrasive papers in
paint and rust removal. At one time
the 60- to 80-grit range are best for
there were many dip-strip tanks in the
this purpose, and leave panel surfaces
United States. Unfortunately, some of
that do not require as much cleaning
the operators of these facilities dis-
Disc sanding works well to remove
up to remove abrasive gouges.
posed of their spent chemicals in irre-
paint and rust from body panels. You
There are many other varieties of
sponsible ways, like dumping them
have to be careful with this technique
spinning devices—abrasive paper flap,
into city sewer systems. Since much
not to overheat metal and warp it.
wire, and mesh-pad-type wheels, for
of the paint dissolved in these efflu-
Done properly, disc sanding is a slow
example—that will clean metal. Some
ents contained lead, many of these
metal-cleaning method.
of these can work on 41⁄2-inch grinders,
facilities ran into regulatory prob-
and produce very good results. Some
lems, and were forced out of business.
strippers will work, and aircraft-type
of the plastic mesh discs sold for this
There are still some dip strippers left,
paint remover is a particularly good
purpose work surprisingly well for
and, depending on your distance
way to do this job.
paint and surface rust removal. One
from one of them, this technique
Disc sanding with abrasive paper
great advantage that abrasive papers
may be a good way for you to cleanse
mounted on a 7- or 9-inch electric
mounted on larger disc sanders (say, bodies and panels to bare metal.
38
AUTOMOTIVE BODY WOR K AN D R UST R E PAI R
CLEAN I NG, MODE LI NG AN D CUT TI NG
My experience with dip stripping
metal. But they must be used slowly,
tells me that this service can vary
due to their potential to warp metal.
greatly in quality and effectiveness
This means a combination of a
between vendors. Some systems use
numerically
low
nozzle-to-target
heated chemicals. Many of them use
angle, low blast pressures, media-rich
electrolytic action to enhance clean-
mixtures with air, and/or long nozzle-
ing. Some operators of these services
to-target distances.
are more careful about the chemistry
Any of these approaches, or com-
in their tanks than others. In sum, if
With considerable patience, abrasive
binations of them, makes the blast
there is a reputable dip stripper rela-
blasting works well to clean sheet
removal of paint and rust from
tively close to you, you should con-
metal thoroughly. You have to be
sheetmetal very slow. However, if
sider this method of cleaning your
careful to work slowly, or you will
you are willing to exercise the
metal panels. In most cases, stripping
warp the metal. Media-rich
restraint to do this, which is often
a single door or decklid does not jus-
air/abrasive mixtures, low numerical
very difficult, you can blast sheet-
tify driving any great distance to a
blast angles, low blast pressures, and
metal clean with good results.
dip-strip tank. On the other hand, if
long nozzle-to-target distances help
The second downside of blast-
you have a complete car to strip, or if
to prevent distortion.
cleaning metal is that it leaves it so
you live near a dip tank, this method
clean that any moisture in the air
may be a valid approach.
repair by what someone thought was
tends to rust it almost immediately.
Abrasive blasting, sometimes
safe blasting. The only way to safely
In dry environments this is a smaller
called sandblasting, is another major
blast sheetmetal is to do it very
problem than in wet ones. In all sit-
category of metal-cleaning technol-
slowly. Soft or small blast media, like
uations, blasted parts should be
ogy. Blasting is a terrific method for
soda and polystyrene beads, are very
worked on quickly, re-cleaned, and
cleaning metal if it is performed by a
slow ways to clean metal. They do
then painted or treated with a preser-
skilled professional. In addition to
not remove deep, pitted rust very
vative immediately.
getting metal metallurgically clean
well, if at all, but they do avoid the
(as the abrasive blasting industry
warping hazard. Harder media, like
Cutting Panel Materials
claims for some blast media), this
silica
sand,
tungsten
carbide,
approach leaves an excellent tooth
aluminum oxide, and cold chilled
There is no perfect, or best, way
on finished metal. This means that
copper slag, will produce very clean
to cut sheetmetal materials. Different
the surface will have nooks, crags,
and crannies that will help paint to
If you are lucky
mechanically adhere to it.
enough to have a
There are two downsides to abra-
good jump shear
sive blasting to clean autobody panel
like this one, you
metal. The first is that blasting tends
can make straight,
to expand surfaces to which it is
clean cuts in
applied. Media, like glass bead, do
sheetmetal almost
this in the extreme. When you
effortlessly. Hand-
expand one side of a surface, but not
cutting techniques
the other, the metal tends to bend
for this kind of job
around the unexpanded side. Put
are never quite as
simply, it will warp. And, no, you
good.
cannot blast the other side to bring it
back into proper shape. I wish that it
was that easy.
I have seen fenders and doors
ruined beyond the possibility of
AUTOMOTIVE BODY WOR K AN D R UST R E PAI R
39
CHAPTE R 4
A stationary hand shear, like this
Beverly shear, is a great way to cut
very accurate shapes into flat
Aircraft snips are great for making
sheetmetal. This type of shear is
accur
ate cuts in flat sheetmetal, and
throatless, so there is no limit to the
work well on mildly curved surfaces.
depth of cut that you can make with it
They cut reasonably quickly and are
Band sawing is a great way to make
into panel steel.
relatively inexpensive.
accurate, smooth-cut lines in flat
sheetmetal. Unfortunately, a good,
metal-cutting band saw is an
expensive piece of equipment.
methods of cutting metal work best
in different situations.
If a straight line is required, it is
best to shear flat metal in a metal
shear. A stationary or hand shear is a
very good approach to delicate detail
work. Another possibility is using
pneumatic and electric shears of vari-
ous types. Tin snips and aircraft-type
snips are good for small shearing jobs
in flat and curved metal. Band sawing
is a very good way to cut flat metal
stock very accurately and quickly.
Unfortunately, good metal band saws
Air-driven saber saws are cheap and are capable of making accurate cuts in
are quite expensive.
contoured sheetmetal. They are the body shop standby for excising diseased
Reciprocating pneumatic saws
areas in panels.
offer a very effective and inexpen-
sive way to cut detailed lines in pan-
builders. Electric hand shears are
use because they spew crescent-
els, and are particularly useful for
also very accurate and fast for mak-
shaped metal fragments from their
cutting bad metal out of large, con-
ing simple cuts.
cut paths, all over creation. Find
toured panels. High-speed abrasive
Nibblers, both air and electric,
some use for those fragments, and
discs mounted in air grinders are
have some application to cutting
you may become wealthy and appre-
another way of making accurate
autobody metal, particularly when
ciated by many. Hole saws and
cuts. This method is favored by
there is no direct entry to begin a
punches are also options for opening
many body shops and custom
cut. Unfortunately, they are messy to
up areas, so that you can use other
40
AUTOMOTIVE BODY WOR K AN D R UST R E PAI R
CLEAN I NG, MODE LI NG AN D CUT TI NG
Carbide burrs mounted in air die
grinders, like the angle grinder shown
here, are a terrific way to fine tune
High-speed abrasive disks, mounted in air die grinders or muffler cutters, are
metal edges for fit-up to other edges.
great for cutting into contoured panels, but are pretty much limited to cutting
These tools are surprisingly
straight lines.
inexpensive.
This type of electrically driven hand
shear cuts metal quickly and
accurately, and is not limited to flat
Plasma arc cutters cut very quickly, but lack the accuracy of mechanical
surfaces. Air-driven versions of this tool
methods of cutting metal. They work well on curved surfaces, and leave
lack sufficient torque, and do not work
weldable edges. Despite the intense heat that they generate, they cause little
as well as electrically driven versions.
distortion in metal because their cutting heat is contained very locally.
AUTOMOTIVE BODY WOR K AN D R UST R E PAI R
41
CHAPTE R 4
cutting methods when interior cuts
in metal panels are required.
Plasma arc cutting is a high-tech
method of severing sheetmetal. It is
very fast, and leaves edges that are
still weldable, unlike cutting with an
oxy-acetylene torch. However, the
lack of fine control of plasma-arc cut
lines restricts the use of this method
to situations where fine accuracy
along cut lines is not required.
Fine adjustments to metal edges
These wire patterns almost look like art, but are really forms that are
are often made with die grinders and
incredibly useful for checking metal as it is formed into faithful replicas of
files. This is particularly true when
original parts. They are also quite durable.
parts are being fine-tuned for tack
welding and welding. High-speed
the actual forming process. In either
electric and pneumatic die grinders
case, these models and forms serve as
are also very useful for making these
standards against which to check the
kinds of fine adjustments.
fidelity of the metal that you are
forming. Sometimes, the metal is
Getting Shapes and
intended to replicate some original
Contours Right
part, section, or feature of a vehicle.
Other times, it is designed to guide
There are many ways to create
the construction of a custom part or
three-dimensional models and forms
panel for an entirely new fabrication.
for sheetmetal objects. These may
Very advanced prototype shops
vary from those that just indicate
have computer-driven systems and
dimensions to be followed, to those
hardware that can sense existing or
that can be used, almost like dies, in
conceptual shapes, turn them into
Paper templates can be made as
numerical coordinates, and automat-
drawings or as three-dimensional
representations of original metal. This
materials or shave them out of
paper model is a combination of
blocks of clay. This kind of capability
drawing and contouring to capture
is wonderful to behold, but presently
the shape of a part that will be
it is way beyond the scope or needs
formed in metal.
of most sheetmetal projects that
don’t involve automobile prototyp-
ers have developed various modeling
ing and manufacturing.
methods in paper, both with flat
My favorite method of transfer-
drawings and by forming the paper
ring most simple contours and
into three dimensions. Clay and
dimensions from existing shapes to
wooden-buck modeling are steps
models is to use number-12 copper
beyond that.
Good, old metal files are great for
electrical wire, or grids made from
When I want to model all of a
straightening cut lines in metal edges,
this wire. Examples of this technique
shape in three dimensions, I usually
and for removing burrs and other
are shown in Chapter 3 and Chapter
use the old egg crate modeling con-
imperfections. They can be used in
12. Every shop has its own favored
struction. It is relatively quick, easy
combination with handheld edg
e
methods of modeling shapes. Wire
to do, accurate, and inexpensive in
deburrers to produce very clean edges.
forms are a favorite with many. Oth-
material costs.
42
AUTOMOTIVE BODY WOR K AN D R UST R E PAI R
CLEAN I NG, MODE LI NG AN D CUT TI NG
Making an Egg Crate Pattern/Model
We created this egg-crate
pattern to physically
represent the dimensions
of the hood scoop shown
in the next photograph. It
is useful for guiding
metal forming to the
correct dimensions. The
footprint diagram of the
The actual shape that will be
part and its recorded
transferred to, and cut out of,
measurements are to the
each rib is captured with a plastic
right of the model.
profile gauge, for transfer to each
rib. This technique results in a very
The strange-looking items in the photo between the two is a matter of personal accurate pattern.
are patterns for the hood scoop on a
preference.
1953 Aston-Martin DB2. This technique
In making this form, the first step was
of creating a three-dimensional model
to lay out strips of 1⁄4-inch tape in a grid along
provides a pattern that is useful as a
the hood scoop. Note that the tape posi-
guide and check for fabricating this
tions were identified by letters and numbers
shape. As the shape is formed in metal,
to keep confusion to a minimum when the
the pattern can be laid over it to check fit,
contours that they represented were trans-
and to indicate changes that need to be
ferred to cardboard and assembled as an
made. This type of pattern also can be
egg crate. The marked edges of the tape
Poster board is an adequate
made in a convex format, in contrast to
(forward or rear edge, and left or right edge)
material for making this kind of
the concave format shown. The choice
were marked to indicate which edge was
model. It has the advantage of being
used, and became the indexing points for
very easy to cut accurately. Some
the parts of the egg crate model. Dimen-
plastic materials are a little stronger,
sions of the footprint of the scoop were
and metal is even more durable.
taken and transferred to a sheet of paper.
A profile gauge was applied to the
indexed against the front of the hood
contours of the scoop, and the profiles
scoop to confirm its exact height as com-