Hammer Welding 101 for Metal Repair

You’re only as good as your tools, so it pays to buy the best you can afford; it really does make a difference. Here are the basics: one body hammer, a flat dolly, a curved dolly, a spoon dolly, a straight edge, and some shears. These items are available from professional supply houses, or you can order them from The Eastwood Company. The patch itself is from C.W. Moss.

To begin, the offending area is sandblasted clean to reveal any metal that needs to be removed.

Burch test-fits the patch panel to determine where it needs to be trimmed for a proper fit.

Because the panels come oversize, you need to trim them on just about all edges to fit snugly to the body for marking.

The top edge of the patch panel is marked onto the cowl so that Burch knows how much of the cowl he has to play with.

You want to cut out as little of the original metal as you can to retain integrity, so scribe a line about half an inch above the nearest rust hole and use this as your cut line.

The first cut is always the hardest, but don’t sweat it. There’s no need to be perfect just yet because you’re going to cut below your guide line, leaving plenty of meat so you can get it perfect with your shears.

While you’re at it, cut out any bracing behind the panel that might get in your way.

Now comes the critical part of this job: panel fit. You absolutely have to get the edges of both your original panel and the patch panel to line up perfectly, or at least as close as is humanly possible. Invest time and effort in this stage, as it makes the whole job a lot easier.

You can also use this time to clean up any rust not reached by the sandblaster. Burch uses 3M stripper discs to make short work of it.

Burch fits the patch panel to the gaping hole in the precious ’32 sheetmetal.

It’s larger than the hole and sits on top of it so he can reach around to the back of the panel and scribe a line for cutting.

Now it’s time to trim the patch panel. Again, it’s important to be as accurate as you can and then some. Trim outside the line...

...and work up to it with a file. If you’re beginning to think this is a little too much like hard work, that’s because it is. But it’ll all be well worth it.

Straighten your edges with a hammer and dolly and fit the panel to the car. If you’ve been doing as you’re told, then you’ll probably be very surprised to see two panels that actually look like they belong together. Don’t get too complacent, though. Check that gap thoroughly, mark your high spots with a pencil or marker, and carefully file it to perfection.

File, check, repeat. File, check, repeat. And so on until the gap is even and only wide enough to let a sliver of light through. What you’re aiming for here are two pieces of metal you can butt-weld together using no filler rod. Yeah, we know it sounds like a pipe dream, but you’ve got to aim high.

Now the fun begins. With a zero-size tip on your torch and a neutral flame, you’re ready to go. Burch begins at the doorjamb end and attaches the patch panel with one tack-weld. Notice that the other end of the panel hangs down lower and there’s quite a gap between it and the cowl. This is to allow for heat expansion of the metal as he tack-welds from right to left. Experience and practice will let you know how big a gap each job requires, but in this instance Burch left about a 1/4-inch gap.

The hammering starts with your second tack. Have a helper handy to hold your gas torch or arrange for a safe place to set it down while you pick up your hammer and dolly and go to work.

As the weld cools, the metal wants to shrink, so your job is to counteract the shrinkage and return the metal to its original shape. As soon as you finish the tack, position your hammer and dolly directly over the center of the weld and start hammering on-dolly. Don’t overdo it, though—you’re not hammering a nail into cement. Let the weight of the hammer do the work, but work fast. The whole procedure only lasts for about 15-20 seconds from beginning to end. You’ll eventually get the feel for when the metal has moved enough.

Give it a couple of minutes to cool. Stretch your soon-to-be-aching back, and off you go again. Just repeat the procedure until you get to the end of the panel. Tack, hammer, cool. Tack, hammer, cool.

Notice that before and during each tack, Burch holds the free end of the panel in place. This helps to encourage the panel to travel in the direction he wants it to go. If you gauge it right, by the time you get to the end of the panel, your gap should be all but non-existent.

Now the entire upper edge is tacked in place, but you’ll notice the sides and bottom haven’t been welded. That’s deliberate, as it leaves the panel free for any tweaking that might be needed as the job progresses. Although it’s almost impossible to show in the photos, it’s absolutely remarkable how much the metal returns to its original shape after a little hammering. Just remember to take it nice and slow. You don’t want to overdo it or you’ll end up stretching the metal too far and you’ll have to start shrinking it, and that’s a whole other kettle of fish.

Now for the final welding. Just take everything you’ve practiced doing your tack-welds and expand it into 1-inch stretches. You want to achieve a nice, even heat range in your welds. This will be evident by the discoloration of the metal. The pattern should be a similar size for the entire length of the weld.

Remember: Weld an inch, put down your torch, pick up your hammer and dolly, and away you go. Work your way back and forth along your weld with your hammer and dolly until you feel you’ve done all you can, and then let it cool.

By the time you get to the end of your weld, you should be in a groove and getting that even heat range in your weld. The key is to be consistent so you get as little distortion as possible. Then just keep on hammering. Don’t feel restricted to dolly on the inside and hammer on the outside. Do whatever you have to do to get the desired effect.

The desired effect is this: a practically distortion-free panel and a weld with next to no filler. You’ll notice Burch was holding a welding rod in the shots, but he only dips it into the molten puddle when absolutely necessary.

After welding the vertical ends in place, it’s time for a little cleanup to reveal...

...a nearly invisible weld.

Like everything else in this world, there's a right way and a wrong way to handle metal repairs. For instance, if you're installing a patch panel into a lower cowl as we're doing here, then hammer welding is the right way to do it so that the patch literally becomes one with the original panel. Practiced at the high end of the automotive repair business, it's a little more time consuming than your average patch-up, but don't let that put you off. If it's good enough for a Rolls Royce, it's good enough for your project. With practice, your efforts will result in a weld that your buddies will be hard-pressed to spot, and not because you slathered it in Bondo and high-build primer before they arrived. A good hammer-weld will bond two panels using next to no filler rod, and the seam will all but disappear when you run the sander over it. Don't believe me? Just watch as Kerry Burch at Limeworks Speedshop wields his magic wand.

Although not for the inexperienced, hammer welding is still just welding, so don't be intimidated. If you're confident with your gas torch, get at it. By the time you're halfway through your roached project body, you'll be surprised at how good you've become. Just remember: Think ahead, anticipate the metal, and most important of all, take your time.