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Amateur Auto Restoration
(Originally published in the Buick Bugle magazine, July 2005)

Part Five: Rust Repair

If you’ve spent any time at all working on an old car, you’re probably very familiar with rust. Rust repair is by far the number-one problem an amateur faces when restoring an old car, and how it is repaired can make the car like new or destroy it forever. As a restorer, I’ve always felt that it was my duty to try to get each component as close to original condition as possible, whether it will be visible or not. Rust repairs are the most visible manifestation of this philosophy—there’s definitely a right way and a wrong way. It is also a very intimidating prospect if you’re unprepared for the magnitude of the task and just starting out. We’ll talk briefly about the wrong way to repair perforated sheet metal and then discuss a better way.

Think of rust as a living, growing cancer on your vintage vehicle—you cannot ignore it, cover it with some miracle paint and hope that it won’t get any worse—it inevitably will. In the process, it’ll destroy everything else you’ve placed on top of it, including that shiny new paint job. With some modern urethane paints costing as much as $1000/gallon, do you really want to risk it? Some rust can be treated and removed by sanding and/or chemical means, and if the metal underneath is solid and smooth, there’s no need to cut and weld new metal in place. Unfortunately, with sheet metal this is rarely the case. Once rust has gotten a toe-hold in sheet metal, it’s often too late to bring it back because the pits that rust creates will leave the metal too thin to be saved—or worse, perforated.

This is what perforated sheet metal looks like. For this article, we'll
focus on the smaller triangular patch outlined on the right.

What is the wrong way to repair rust damage in sheet metal? As far as I’m concerned, it is any fix that doesn’t involve cutting it out and replacing it with fresh metal. More than a few body shops take the “out of sight, out of mind” approach, using paint and plastic filler to compensate for inferior rust repair work. Cars that have been repaired using cut-rate techniques and materials are usually easy to spot, with wavy body panels, pock-marked rockers and inevitably, cracked paint. Careless workers grind out most of the rust with a grinding wheel or sander, throw some fiberglass tape over the hole, cover it with filler, sand it smooth, then paint it. This is how you fix a hole in drywall, not sheet metal! Too many smart people have been convinced that this is the right way to do it by a smooth-talking guy in a body shop, but it’s total hack work and a temporary solution at best. Filler certainly has its place, even in a top-quality restoration, but it isn’t designed to fill large holes. Don’t be fooled if you are paying someone to repair the rust on your collector car, and don’t think it is the easy way out if you are doing it yourself. No good will come of it.

Properly repairing rust is not complicated—just cut out the old and weld in the new. Yes, it takes some specialized equipment like a welder, and it will definitely require practice to learn to join thin sheet metal, but it is a skill at which any amateur restorer can become reasonably proficient. There may be some tasks best left to professionals (recreating complex shapes or large panels that are not reproduced, for example), but if your car is fairly solid, then the repairs to commonly rusted areas such as rocker panels, door bottoms, inner fenders and floors can be done at home by an amateur with professional-grade results.

Let’s walk through the process:

1. Cut out the rusty metal. How? I use a variety of tools ranging from a cut-off wheel to my electric reciprocating saw (Sawz-All) to a small hand hacksaw. Study the area carefully before you start cutting because there could be braces, hoses, wires or other items underneath the area you’re patching. Make sure you are only cutting what you want to cut.

Be careful to only cut the rusted metal, not any braces or lines
that may be underneath!

I recommend cutting in straight lines. Round shapes are much harder to cut out of the replacement material, and make getting a tight fit more difficult. You will often find that it is necessary to make an unusual shape, but if you keep the lines straight, it will be much easier to achieve a tight fit with your patch.

Finally, don’t be afraid of cutting your car. I know it’s intimidating at first, but if you plan carefully, it’s rare to have a catastrophic failure when cutting metal from an isolated area. Well, I suppose there was that one time in high school where we cut the roof off a buddy’s Ford Maverick to make it a convertible and later had to weld it back on at the behest of his father, but I digress.

2. Make a template. The most important step in making a patch panel is the template. I use poster board to create a flexible, easy-to-cut template of the patch area. The easiest templates to make are those where you can hold the poster board up to the hole, trace the hole onto the template, then cut it out. This will leave you with an outline that you can transfer to your sheet metal and cut the patch. Of course, there will be times when you can’t simply trace a hole, and that’s when you’ll need to get creative with your template. Just make sure to get it as accurate as possible in cardboard and the metal patch will be that much easier to make. This is not a place where “close enough” is acceptable—take the time to make your template as perfect as you can before you start cutting metal.
    

3. Make your patch panel. Your patch panel should be made of the same gauge metal as the rest of the repair area—you don’t want to weld 16-gauge steel into a 20-gauge panel because it will be difficult to achieve adequate weld penetration and a uniform surface. Sheet metal is inexpensive (I bought a 4’x12’ sheet of steel for the floors and trunk of the Century for less than $50), so get the right material for your application. Talk to your supplier about some options because there are types of steel specifically made for custom metal shaping. If possible, show him a clean sample from your vehicle’s body so you can determine the proper gauge.         
   
   It’s rare that you will have a perfectly flat patch panel—inevitably you’ll have to bend, fold, twist, stretch or contour it in some way. This is the most time-consuming part of making the patch panel, and you will have to get creative. I have a collection of hammers, dollies and chunks of metal that all offer unique shapes over which I can hammer the sheet metal to get an appropriate curve. My most frequently-used metal shaping tool is my bench vise where I can clamp the sheet metal patch between pieces of metal I’ve ground to the desired profile, then bend or hammer the patch over it, creating the desired shape. It sounds more complicated than it is—the bottom line is that you will have to innovate when trying to make curved panels. I have used everything from the handle of a garden rake to an old wheel from a floor jack to create curves in sheet metal.          
   
   Working with metal is difficult, no two ways about it. It will take time to develop your skills, and at first you will likely scrap more patches than you actually weld into the car. This is completely normal for a beginner so don’t get frustrated when things don’t immediately go the way you planned. Metalworking is a skill that can take a lifetime to master (though you can probably get very good at small patches very quickly). Be patient, check your patch frequently by holding it up to the hole and noting where it needs to be adjusted. Write on the metal, make markings, draw arrows, whatever you need to help you move the metal around accurately.

Getting the patch to fit is by far the most difficult part of this process and if I can do it, you can do it, too. All it takes is practice. Always remember that the restoration process is a journey you should enjoy, not merely a destination.
 

4. Tack the patch in place. Once you’re satisfied with the fit of your patch, it’s time to weld it in. Like metalworking, welding is a skill that takes time to master. I recommend that beginners invest in a moderately-priced MIG welder from a well-known manufacturer. Don’t bother with cheap flux-core welders, get a real MIG with a gas regulator and use high-quality wire. This is not the place to start cutting corners. With a decent MIG, you will probably be able to lay down passable welds after only an afternoon of practice. I won’t go into a lot of detail on welders and which brand is best—you can find more of that information on the Internet or at your local library or trade school.

Patch tacked into place. Note the very tight gaps and scrupulously
clean metal. I used a hammer and dolly to flatten the patch and
surrounding metal.

I will say that no matter which type of welder you choose to use, practice first. Get some scrap metal and practice welding just to get a feel for what makes a good weld. Start with heavier stock so you don’t have to worry so much about burning through or warping the metal while getting a feel for the process. Once you’ve got some competency there, practice on some sheet metal that is the same gauge as the metal you will be working with on your patch panels. Do experiments so you know how much heat you can apply before you burn through or how long a bead you can run before the metal starts to warp. I had a professional welder where I work give me some lessons during lunch that were especially helpful—you may want to consider taking a class or seeking out a experienced hobbyist for some advice. Practice is the single-best thing you can do to improve your weld quality—no book or website can teach you what an afternoon of practice will.  

There are several kinds of joints you can use when welding fresh metal into a body panel, but only one that I feel is acceptable for restoration-level work: butt-welding. A butt-weld is where the patch is exactly the same size, shape and contour as the metal you removed and will fit with only a very small gap around the perimeter and no overlap. With a tight gap and meticulously clean metal at the joint, welding will join the two pieces of metal as if they are one.   

This is in contrast to a lap joint (most likely derived from the word overlap), where the patch is larger than the hole and is simply placed over the hole, welded around the edges and covered with filler. Lap joints are perfect places for new rust to start because they are just about impossible to seal completely. They are also very difficult to hide because the two pieces of metal are stacked on top of each other like a sandwich instead of being invisibly knitted together at the edges. Lap joints can be flanged to help hide the patch, but unless they are replicating an original lap joint, try to stick to butt welds for patches in the middle of a panel. Butt welds are a lot more challenging, but remember that making an invisible repair is your number one goal. You just can’t hide a lap joint.

I always tack-weld the patch into place in a few areas around the perimeter. A one-second blast with the welder is typically enough to knit the metal together and hold it in place while you work—you may be surprised by how strong a tack weld can be. Four or five such tack welds will hold the patch in place solidly but still give you opportunity to make any last-minute adjustments, either with a hammer and dolly or with your cut-off wheel should you need to remove the patch and start over. VERY IMPORTANT: Do not finish weld the patch until you are satisfied with the fit.
 

5. Finish weld the patch. Once you’re happy with the way the patch fits and you are certain that it will be flush with the surrounding metal, go ahead and weld it in permanently. Sheet metal is challenging to weld, but you’ll have more success if you remember a few tips:
    

·     Control the heat. Sheet metal will warp very easily if you apply too much heat. With a MIG welder, most warping issues are caused by welding too long a seam without taking the time to let the metal cool. For reference, it usually takes me about an hour to weld a 12-inch seam using the technique I’ll share with you below:   

The secret to successfully welding sheet metal is to do a series of spot welds and skip around the perimeter of the patch so that heat doesn’t build up in any single area. This technique doesn’t leave a pretty weld, but it does make for a better patch with no warping. Use a slightly hotter setting to ensure good penetration, and hold the trigger for about a second. If you’re doing it right, your welds will be flat with very little wire build-up on the surface. Make sure you take frequent breaks to allow the metal to cool (never use water or wet rags to rush the process unless you know what you’re doing!). If you warp the metal, it is VERY hard to repair, so take your time. Always remember exactly why you’re doing this. I find that I have the most success by aiming at the “stronger” metal and pulling the weld pool towards the “weaker” metal (i.e. starting on the fresh metal patch and pulling the weld pool towards the older metal in the car body).   

     
Spot welds aren't pretty but minimize distortion of the panel.
Note the “heat zone” visible around the welds.

·     Don’t “chase” your holes. Inevitably, you’ll blow through the metal as you weld. Your first instinct will be to try to immediately fill the hole with more welding (and this will be a very hard instinct to resist, I assure you). STOP! All you will do is make the hole bigger. The area is already overheated, so adding more heat will only create a bigger hole and little metal stalagmites hanging below the weld area. Come back later and try to fill the holes with a series of quick spot welds around their perimeter. If necessary, cut out the area and make another little patch. Do not chase your holes. This is very important, so I’ll say it again: DO NOT CHASE YOUR HOLES. 
 

·     Stop frequently to use a hammer and dolly. Keeping the two pieces of metal flat and even with each other is critical. As you weld, they may move around and distort. Use a hammer and dolly to keep everything properly aligned. If you are not familiar with hammer and dolly techniques, invest in some inexpensive tools and practice. It will become one of your most useful metalworking skills.

·     Keep your welding tip close to your work. When I started welding, I tended to keep the nozzle 3/4” or more away from the weld area. While this was adequate, I often found that my welds tended to be too shallow, didn’t get full penetration and were often contaminated and porous. To remedy this, I slowed the wire feed speed on my welder and kept the nozzle extremely close (1/8”-1/4”) to the working surface. Position the electrode, pull the trigger, and ZAP! the weld is done. Move to the next area and repeat. Use only as much heat and speed as you require to achieve adequate penetration. Any welding wire deposited on top of that will only have to be ground off later, and welding wire is extremely tough.  
 

·     Watch what you’re doing. You might be surprised by how many welders don’t watch what they’re doing and allow the weld to just be “close enough” to the target area (I’m sure we have all seen the guys on TV who close their eyes and pull the trigger). This is especially true with MIG welders, where the large nozzle can partially obscure your view of the weld pool. With a good welder’s helmet (and I emphasize GOOD—invest in a high-quality name-brand helmet to protect your eyes), you should be able to look right at the weld and see exactly what’s going on. I learned to watch from the side so the nozzle doesn’t block my view.
 

6. Dress the weld. This is where the quality of your weld will show: when you grind down the weld bead to create a single, flat piece of metal with a virtually undetectable repair. I used to use 36-grit sanding discs on my angle grinder to just grind away at the weld seam, but have I found a better way. Now I use the edge of my cut-off wheel to concentrate only on the weld bead that protrudes above the metal’s surface (and it may be considerable). I usually grind the bead down to within 2-3 thousands (.002 to .003 inches) with the wheel, hammer it flat using a hammer and dolly, then finish with a sanding disc. For thicker beads, stack two wheels for a wider footprint. The cut-off wheel makes quick work of the weld bead and because the wheel is so narrow, you can avoid the surrounding metal entirely (unlike the sanding discs which can destroy the surrounding sheet metal by grinding it too thin). Use caution, work slowly and again, WATCH WHAT YOU ARE DOING. Get a face shield, not just safety glasses, because a lot of sparks are going to be coming at you and you’ll want to be close to your work. Protect your face, even if it’s as homely as mine.

Weld bead ground down using cut-off wheel.

Once you have the weld ground nearly flush with the surrounding surface, get your hammer and dolly out one more time and hammer the heat zone flat. You can recognize the heat zone as a bluish ring around the weld bead (see Figure 3). If you hammered while welding the patch, this will not take much force, just a few taps to get the new and old metal in line with one another. The welded seam tends to stretch away from the welder nozzle, so you’ll want to gently coerce it back into place and flush with the surrounding metal.

With everything pretty close to flat and flush, that’s the time to take your sanding disc and lightly finish grinding the area smooth. Evidence of the patch should be slight—some grooves and pockmarks where the weld wasn’t perfect are normal. Don’t worry about making it perfect with more welding. If you need to do some additional hammer and dolly work, do it now. Just don’t overdo it with the sanding disc, which can remove too much material very quickly and put a lot of heat into the patched area.         

If, for cosmetic reasons, you feel it is necessary to make the patched area perfect, you can always finish the patch with a little lead or plastic filler before you paint (this is the correct application for body filler). If you did your job well, the repair will be invisible and permanent after paint is applied and nobody will be able to spot it. 

Final patch after being sanded flush with the surrounding metal. Note
that the original stiffening bead has been preserved.

Remember that rust repair is a job you can handle. People have been doing it for decades, many of them no smarter or more talented than you. Restoring sheet metal can be frustrating and intimidating, but it is also very rewarding to see the finished results. You will also save a pile of money, even considering the purchase of new equipment. Professionals who specialize in this work are not cheap: once you see the labor required, you will easily understand why metalsmiths charge what they do and why professional restorations are so expensive.

Next time we’ll talk about polishing stainless steel and aluminum trim, the chrome-plating process and getting your car’s bright work to shine like new.