Welding Aluminum Body Sheet Part 3

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Welding Aluminum Body Sheet Part 3

Adhesion Coatings and Fillers

So we've straightened and welded the aluminum-now how do we get the coatings to stick? Back in the early '70s, before the tested, comprehensive finishing systems of today, it seemed to he both a crap-shoot and a big chore to get an aluminum car painted. One of the popular sayings regarding fillers that I heard hack then was: "If it don't stick the first time, it'll always stick the second." Oh, boy! Real science.

It took a while for me to figure out that aluminum body sheet oxidizes very quickly, so I wasted my time cleaning and sanding the night before I would be applying filler or primer. I finally narrowed the process down to cleaning the aluminum a maximum of 20 minutes prior to coating. When I kept to that maximum, I had no more problems with adhesion.

By 1983, I had a great system for doing extremely valuable vintage racing cars:

(1) Finish all the metal work.
(2) Solvent-wash and spray with a catalyzing etching primer over all the different metals.

Then I found a really hard etching urethane primer over which we could apply small amounts of plastic filler. This discovery allowed us to avoid sanding the .050 and .040 thick aluminum bodies down to .020 with 240, 320 and 400 sandpaper.

In the first installment of this series, we showed you a 1961 Aston Martin DB4 which had been hit and was in the process of being restored. This is the finished product as it appears today. The left front and door were repaired using the original parts and no filler.

By 1984, we had refined the hottest secret system going and were talking to paint reps and in-house chemists all the time, trying to stay right on the edge of the straightest, fastest, non-shrinking and most durable paint jobs any race car could have.

Now our trailblazer path has become a four-lane turnpike, with the official system teams in place today including etching hard primers, super fillcoats, and best of all, great color matching capabilities.

Here's a myth that has been "de-mythtified": The heat from plastic filler warps panels. Why does the application of plastic body filler cause some flat panels to go down? If the panel has been worked and/or ground down to the extent that it is somewhat loose, weak or lacks support, the contractive power of the plastic pulls the top side in on itself driving it down.

On a hood or lid, I try to shim the framework a bit for support, or I sand the backside and apply filler there, too, so as to balance the contractive forces. Many body men have told me that they thought it was the heat of the plastic that warped the panel. However, I tested both the contraction and heat theories, and eliminated the heat theory by applying the plastic at 34 degrees F. with excess hardener to compensate for the temperature extreme. The heat from sunlight, however, while greater than that of catalyzing plastic, has no such warping effect on the same panel.

Alodine/Chromate

Long a tradition in aluminum primers, zinc chromate is still employed in aircraft production. The only drawback to this type of primer is the potential for excessive film thickness. Application should be heavy enough to color somewhat- yet not color completely- because water absorption into the film coating can occur with heavy coats. Carefully follow the manufacturer's instructions for the use of their product.

Alodine as an industrial process is reliable and durable. However, for hoods, fenders, etc. being repaired in a shop environment, I have not found it completely reliable. There may be some alodine product we haven't tried, that would allow efficient application in the shop and could be reliably overcoated with primer, filler and surfacers. But, generally speaking, 'If it doesn't stick the first time, it will the second" with the alodines we've experienced.

Sources for Continuing Education

There's nothing like a good shot of adult education for picking up welding skills. High schools and community colleges have great programs for MIG, TIG and perhaps torch basics. Aluminum is certainly not hard to weld-I actually prefer to steel.

More information can be obtained from Publication AT1 from the Aluminum Association. Published in March 1994, the 16-page booklet is titled Repair of Automotive Sheet by Welding. Their comprehensive technical manual Welding Aluminum: Theory and Practice has enough information to solve any puzzle I can imagine, and I would highly recommend their Aluminum Welding Seminar held periodically around the U.S. Contact the Aluminum Association at:

American Welding Society
550 N. W LeJeune Road
Miami, FL 33126
(800) 443-9353

This article is a brief summation of my 25 years doing aluminum body repair, rebuilding and fabrication. All aspects have been thoroughly researched. My apologies for taking a year to get it out- I hope the depth and detail will be of benefit to you.

Kent White received much of his training at Harrah `s Auto Collection where he earned the Master Technician title. He worked on automobiles and aircraft at Harrah '5 before opening his own business, The TM Technologies. He has spent 20 years honing his skills on his own work and by seeking out, and training with, many of the great craftsmen in the metalworking world. TM Technologies offers metalwork mg tools, instructional videos, consultation and workshops. For information and a free catalog call (530) 292-3506 or write TM Technologies at P.O Box 762, North San Juan, CA 95960.

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