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By John Kelly/Ghia
Specialties
When welding a patch panel or
fender flare on a car, I use either oxy-acetylene (gas welding) or wire-feed (mig
welding), in that order of preference. Tig welding is great, but few
do-it-yourselfers have a tig welder, so we'll stick with the first two. Gas
welding is my first choice because the metal stays fairly soft and workable, and
metal-finishing is easier (and more quiet). Mig welding work-hardens the metal
and seems to shrink the weld area excessively. Gas welding also shrinks (heat
shrinks); however, the metal is still workable with hammer & dolly, whereas a
mig-welded panel is too stiff to work effectively.
Gas Welding: As a good rule of thumb, gas-weld the panels you can reach
both sides of; mig-weld where access is more limited. I sometimes do both on a
panel. If you choose this method, be aware that it's easier to mig-weld over a
gas-weld than vice-versa, so do the gas-welding first (at least where the welds
join). When gas-welding, I use the smallest tip I can get away with, usually a
00 or 000, and low line pressures. If the torch pops when welding, the pressure
may be too low, the tip may be too large, or the tip may be dirty. If the torch
is noisy, the problem might be, high line pressure, too small a tip, too much
oxygen, a dirty tip, or a combination of these.
Your weld-puddle should look smooth and glassy. If your weld falls through,
you're too hot; if it takes more than a few seconds to get a puddle going,
you're too cold. If your torch acts up once you're set up the way you like, the
problem is usually a dirty tip. Try welding two pieces of 20 gauge steel
together. Connect the pieces together edge-to-edge (butt weld). If your heat is
right and you have a perfect fit, you can fusion-weld them. Fusion-welding is
basically melting the metal together, without using filler-rod. You can make
beautiful little welds this way. I usually fusion-tack my panels together and
use a little filler-rod when finish-welding, to keep from having a concave
(shallow) weld surface.
Check the back-side of your weld to make sure you're getting good penetration.
It should look like a weld, not two edges glued together. If you didn't get good
penetration, you can fusion-weld over the bad spots from the back. This exercise
will help you make good welds later, when you can't see the back-side of your
work. Remember, heat shrinks, so stretch your tacks with a hammer &
dolly; the same applies when finish-welding. After 1/2" to 3/4"(as you get more
experienced, you may decide to weld several inches at a time), set your torch
down (turned off or in a safe holding bracket), and use your hammer & dolly. The
object is to remove some of the shrinking you've caused by welding, while
keeping your panel in shape. Don't stray too far from the weld to begin with.
You'll find you can get your shape back if you patiently work the weld area
first, and then address any peripheral warpage. Remember, if your panel fits
well to begin with, you should be able to make it fit when you're finished,
without resorting to drastic measures. Here is an excerpt from the directions
that I send out with my shrinking disc. This may help a little with
metal-finishing:
Dent repair and metal-finishing: To repair a dent, use a dolly to bump up
the low spot from behind. Some larger dents are best worked from the perimeter
in. Bumping with a dolly will bring the dent back near the original contour.
This simple step is important throughout the repair, because, in addition to the
inevitable small areas that need minor stretching later in the process, you will
probably find low spots that just need bumping up. After bumping the dent up to
its original contour, start working the metal off-dolly. This means pushing up
with a dolly on low spots while hitting high spots with a hammer or slapper.
This will start to get the panel smoother. Now start some medium-force on-dolly
work. Usually, on-dolly work is stretching the metal between the hammer and
dolly but, in this case, very little stretching is done, especially if you use a
slapper instead of a hammer, as the force of the blow is spread more evenly. You
are using multiple hits to planish (smooth) the area. Now check the shape of the
panel. Use templates taken from the same spot on the other side of the car
wherever possible. Use one up and down, and another front to back to see where
the shape is too low or too high. Sometimes the whole area will still be too low
and need more bumping and hammer-and-dolly work. Once you are satisfied that the
general shape is right, you can start to pick up specific low spots by
stretching on-dolly. Use a dolly that has a slightly higher crown than the panel
being worked, and a hammer with a slight crown in it. This way there is a small
contact area between the hammer and dolly, making it easier to stretch small
areas up. You must push up fairly hard on the dolly. You should see small marks
on the metal where it is stretched by the blows. Lightly file the area to show
the highs and lows, then repeat the hammer-and-dolly steps, and file lightly
again until you have the whole area smooth but a little too high. As an
alternative to stretching up the low spots with a hammer and dolly, a tool
called a bullseye pick [available from Ron Covell on my links page] can be
modified to work very well. It is a little easier to use and may be easier on
your arms if you have a lot of work to do. The tip must be ground down so that
it is not so sharp, otherwise it will damage the panel. A tip I picked up from
Wray Schelin: During metal-finishing, use a large magic marker ("Magnum" size)
to ink the whole repair area before filing. This really makes the low spots
stand out, just like using a guide coat for sanding primer. For more about
metal-finishing from a different perspective, please see the Jag Lovers articles
written by Wray Schelin, also on my links page. "The Key to Metal-Bumping" by
Frank Sargent is a good resource booklet as well.
Using the shrinking disc: Once you have the metal smooth but high, start
running the disc over the surface, back and forth, while moving sideways
slightly after each pass - basically a zig-zag pattern, much like what you would
use when conditioning a panel with a sander. For most applications, a 6" x 6"
area of coverage is a good starting point. Small high spots will turn blue. Stop
immediately and use a wet rag to quench and cool the metal. Do not rush!
There is no hurry. I keep a rag in the bottom of a bucket with about an inch of
water in it so it doesn't splash much when I drop the rag in after use. The
smoother the panel is, the longer you can run the disc without turning any part
of the panel blue. It is not necessary to use the disc until the metal turns
blue in order to shrink; use it just long enough so that when you quench it with
a wet rag it steams. This will take practice to gain the experience of knowing
when to stop. Run your hand over the metal both up and down and back and forth
to feel the surface while it is still wet. You will be able to tell where the
high spots are, and use the disc for a shorter period of time to shrink specific
areas. The disc will mark the metal and show the low spots as unmarked. Do not
hesitate to go back to some of the previous steps of on-dolly stretching or
using the bullseye pick to raise low spots. You may find it necessary to bump up
some low spots, or even go back to some off-dolly work. This is part of the
process. Once you have done an operation, never assume that that can't be the
problem. Always let the panel dictate what needs to be done. Most severe damage
will require multiple passes of the shrinking disc interspersed with quenching,
hammer-and-dolly work, and/or the bullseye pick. Once you have the panel nice
and smooth, you can spray a guide coat on it, or use the Magnum marker, then
sand with an appropriate sanding block with 80 grit to help show small
discrepancies. At this stage, you can use a worn-out Scotch Brite pad on a 7"
Velcro backing pad fitted to your sander, just as you would the shrinking disc,
then quench, to simultaneously polish the surface and shrink a little more as
well.
Mig Welding: Most people who have just started mig-welding, seem to have
a hard time seeing the weld as they go. If you're having this problem, make sure
the clear lenses protecting your weld lens are new. Also, try using the trigger
to do a puddle, then let go of the trigger, move the gun slightly, and repeat
this process over and over as you move along. This way, you won't feel like the
machine is forcing you to go too fast. You may find this method helps the
quality of your welds, too.
Don't hesitate to play with the weld settings on your machine; that's what
they're for. Is your gun jumping? ?getting lots of sparks? Your wire speed is
probably too high in relation to the voltage. Blowing holes in your work (even
with the stop & start method described earlier)? ?welds look like lava flows?
Your voltage is probably too high, in comparison with your wire speed, or your
voltage and wire speed are both too high. Lumpy-looking welds? You should be
welding hotter (more wire speed and voltage).
Tack Welding: Take your time, and use lots of tacks; not only do they
hold your panel in place, they also help dissipate the heat evenly. The best
results are achieved when you insert the panel flush with the car body, instead
of overlapping, and less grinding and filler will be required. If you have to
push the panel into place to tack it, you will have more of a problem with
distortion than if you make the panel fit better to begin with. Tack about every
inch or so. If you grind the tops off the tacks, you may have an easier time
making a good final weld.
Finish Mig Welds: When doing your final welding, weld only one small hot
tack at a time. Do not weld in an area that is warm to the touch. The more heat
you put in one place with a mig welder the more leverage the warpage has. When
finished, grind the weld as smooth as possible, then sandblast the area. If you
don't have a sandblaster, carve all the scale out of the welds using a small
broken drill bit held at an angle in your drill motor. You want shiny metal.
When you need filler over welds (usually the case with mig-welding), your first
application should be a fiberglass-reinforced filler, as it is tougher and
shrinks less than regular filler.
Some Final Tips: Use templates on any shape that isn't flat. Take the
time to protect your eyes, ears, and lungs. Keep a fire extinguisher handy, and
keep a fire watch on your shop at least ½ hour after welding.
I wrote this because I was unable to find out most of this information when I
was starting out. I hope it will be helpful to someone. This article states my
opinions and is not the gospel, as I'm sure others may have different ideas when
it comes to some of this stuff. Get to work! Updated March 2003.
Address questions or comments to John Kelly using the email link on my web site.
Please see the "latest work" link for shrinking disc information, and
metal-shaping tips.
www.ghiaspecialties.com
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