For years, the mainstay for paint removal and grinding welds was the 24- or 36-grit
fiber disk on a high-speed air grinder. If you had a large number
of welds to grind or you were welding on the frame of a body-over-frame
vehicle, you might use a 1/4-inch-thick hard grinding wheel. Boy,
would the sparks fly then.
Old habits sometimes die hard – or at least
die slow – but die they must.
Let’s consider, for a moment, the grinding
requirements for the new age of collision repair. First, heat-sensitive
high-strength steels and thinner panel thickness have necessitated
the need for change; second, aluminum has really forced us to
rethink our grinding methods; third, looking for ways to increase
productivity at a reasonable cost is a priority for technicians
To help meet these new requirements, we’ll
take a quick look at what the current recommendations are, examine
several potential problems and solutions, and check out several
new products that may help improve productivity.
The Nitty Grit-ty
For paint removal on modern vehicles, the
current recommendation is a 40- or even 50-grit fiber-backed disk
on a slow-speed grinder. Right away, we have a problem. Most of
the 4-inch air grinders in use today turn at about 10,000 RPM,
and to make matters worse, the 36-grit fiber-backed disc is still
the preferred grit of too many technicians.
What happens when you grind on modern, heat-sensitive,
light-gauge steel with a course disc at high RPM? Two things are
likely to occur. First, if you don’t keep moving, the panel will
overheat. Second, you’re likely to reduce the panel thickness,
sometimes as much as 50 percent, without even realizing it. If
you hit a sharp body line at the wrong angle, you might even grind
To prevent these problems, some technicians
now use either a 6- or 8-inch air sander with a 40- to 50-grit
paper disk. Keep in mind that large-diameter discs improve productivity
on large areas. The problem with soft backup pads and paper disks
is they don’t last as long as the fiber grinding disc.
When selecting air tools and abrasives for
paint removal, remember that slow speeds are better than fast
speeds; finer-grit discs are better than course discs; and a combination
of air and electric grinder/sanders, and 8-, 6- and 4-inch diameter
discs may be necessary to complete some tasks.
An Abrasive Topic
New abrasives have also helped improve grinding
on newer vehicles. One of the most important changes was the type
of grit used to produce abrasive products: The old standby was
aluminum oxide; the newer products are a zirconium grit and a
ceramic aluminum-oxide material, which both offer several key
The zirconium product is self-sharpening,
eliminating the need for heavy grinding pressure – which allows
cooler grinding. Paints are easier to remove without clogging,
and the metal itself stays cooler. The zirconium abrasives do,
however, cost more than aluminum-oxide abrasives.
The newest abrasive material – a ceramic aluminum-oxide
material – should be available this year. This abrasive has self-sharpening
characteristics, is tougher and longer lasting than the zirconium
products and is ideal for heavy grinding and weld finishing.
As you look at the economics of abrasive materials,
consider that the higher cost of premium products is offset by
fewer disk changes. This aspect can offer substantial labor savings.
What to Do About Welds
A couple of tips concerning finishing welds:
First, if the weld will be hidden, don’t grind it. A quick leveling
of the weld will offer an economic method for checking for porosity
and other defects.
When you’re working with fiber-backed discs
and hard wheels, consider this general rule: When using a fiber-backed
grinding disc, hold the disk at a 10- to 15-degree angle to the
work, which will keep a larger percentage of the disc on the work.
As the angle increases from the work, the smaller the area of
abrasive on the work. The disc is more likely to cut in rather
than remove paint or level a weld.
If you currently use a high-speed, 4-inch
air grinder, ever notice that the disc floats up and down on either
side when grinding a weld? Most backup pads are designed to be
somewhat flexible. This works great for flat panels or gentle
curves, but when grinding on a weld that’s higher than the surrounding
panel, the disc moves up and down. It’s very difficult to level
a weld without removing the surrounding metal. Also, ever had
problems with finishing a weld on a complex shape, such as a rocker
panel? A flap disc – discussed later – can help solve that problem.
All About Aluminum
Aluminum requires special methods, so "Think
aluminum." (Also remember that failing to observe the following
recommendations may lead to premature corrosion.)
Probably the most important rule is to never
use grinding and sanding products that have previously been used
to grind steel. Course discs (most car makers are recommending
a 36-grit disc) may be used to grind aluminum welds, but some
weld metal must be finished with a finer-grit disk. The key to
proper grinding on aluminum is to refer to the repair manual for
each vehicle you work on – don’t guess.
Extreme care should be taken to not remove
aluminum from the area on either side of the actual weld, which
reduces the thickness of the base metal. When the base-metal thickness
is reduced, you create a potential weak spot.
Products with Potential
The following products have been used in the
metal-fabrication industries for years and may be worth investigating.
They’ll be more useful when finishing welds – such as plug welds,
lap joints or butt joints – but you’re not likely to see an advantage
for removing paint. The exception is the flap disc, which might
prove useful when removing paint from a complex shape, such as
a rocker panel.
Flexible grinding wheel
– This combines the performance of both the flat-fiber disc and
the 1/4-inch-thick hard wheel. It rapidly removes weld metal like
a hard wheel, and if you lighten up the pressure, it finishes
like a fiber disc. It also works on steel and aluminum without
clogging. But don’t alternate back and forth between steel and
aluminum – it will require the specified backup pad.
This product is available from several manufacturers
in a variety of grits (36, 60, 100), and common diameters are
4 1/2-inch, 5-inch and 7-inch. It costs about $2 more than the
1/4-inch hard wheel and about $3 more than a regular fiber-backed
– This product is made up of small pieces of abrasive adhesively
bonded to a molded backup pad. When the abrasive has been consumed,
you throw the disc away.
Flap discs are about six times more expensive
than a fiber disc, but they cut cooler, last longer and constantly
have new abrasive exposed. They may be used for both steel and
aluminum, are available in 40, 60, 80 and 120 grit with either
4 1/2-inch or 7-inch diameters. (You may, however, find the 7-inch
cost prohibitive for typical collision-repair work.)
Hard grinding wheel (for aluminum)
– The 1/4-inch-thick hard grinding wheel in either a 4 1/2- or
5-inch diameter configuration is a favorite when removing weld
metal on heavier steel. Many techs keep one of these wheels for
weld finishing and for preparing a V-groove when working with
body-over-frame vehicles. This type of wheel is also available
in a special formulation for aluminum. The wheels designed for
steel will load up; the one designed for aluminum does not. (Don’t
forget that grinding aluminum will require more care than grinding
The Daily Grind
Although today’s grinding requirements are
different from yesterday’s, the new products out there do make
grinding easier. At the very least, they’re worth a little investigative
effort on your part. If you don’t like them, you can always go
back to the standard fiber disc.
But remember, as the collision-repair industry
changes, so must technicians and their processes – if not, they’ll
become obsolete along with old repair methods.
Fred Kjeld is a contributing editor to BodyShop
A Second on Safety
Wear approved Z-87 safety glasses with side
shields. Also wear an approved, clear, full-face shield for grinding
and sanding operations. Protect those nearby when you’re grinding,
and also protect nearby painted panels, trim and glass from grinding
- Use the machine guard.
- Use the correct backup pad for the machine and remember that
worn out and nicked backup pads contribute to grinding accidents
and can also reduce the life of the abrasive by as much as 50
- Follow the recommended RPM for the type of abrasive you’re
- Make sure the hole size is correct for your machine.
- Don’t interchange backup-pad nuts from one manufacturer’s
grinder to another. They may have the same thread size, but that
doesn’t mean they’ll fit correctly on the pad. A fiber disc flying
off at 10,000 RPM is a lethal weapon. Also, grinding around rusted
holes can be one of the most dangerous of all grinding operations.
One time I was grinding a rustout, when the disc caught and a
chunk of it went flying off. Later, I discovered the disc chunk
imbedded in the 5/8-inch-thick sheet-rock ceiling. This made a
believer out of me.
- Always follow the recommendations of the car companies, grinding-tool
manufacturers and the abrasive manufacturers.