Last month's installment in our Paint & Body series walked you through stripping old paint off your project car. In the next couple of issues, we're going to get into what to do after you've discovered what the old paint was hiding.
If what you found isn't pretty, your first decision will boil down to the two Rs: repair or replace. If the damage is extensive (say, terminal cancer in the form of runaway rust) or parts are readily available, cheap, and easy to install, you'll probably spend less time and effort hanging new metal than trying to fix the rotten stuff. On the other hand, if your car is rare or obscure enough that replacement sheetmetal isn't available, all you're facing are a few dents and dings, or you don't think your welding skills are up to snuff, then repair is the route for you.
For some restorers, repair is always preferable to replacement because original sheetmetal fits better than repop no matter how good the reproduction is. Plus, there's value in retaining original sheetmetal. According to Richard Boone, "Almost any damage can be repaired if you have the proper equipment and time." Then again, Boone works for The Eastwood Co., maker and seller of just about every tool and repair material known to man.
He does have a point, however; there are techniques out there to remedy just about any sheetmetal malady you'll encounter when bringing a project car back to life. Read on to learn how to reshape dented and dinged panels to their original contours. As you're reading, keep this thought in mind: The best bodywork results come from knowledge and understanding of how sheetmetal can be shaped combined with the right hands-on techniques. The methods shown here appear straightforward enough, but don't expect to get a glass-smooth finish the first time you attempt these techniques. For that reason, all the bodywork experts we spoke to recommended practicing-a lot-on junk panels and fenders before knocking on your project car's sheetmetal.
Hello, Dolly
The most basic method of fixing dents is to beat on the metal until it's back into shape, and there's a huge assortment of hammers and small anvils, called dollies, that will do the trick. The technique only works, though, if you can reach both sides of the dented metal, as the dent will be sandwiched between the hammer blows and the dolly.
While the hammer-and-dolly method of metal repair is the most basic, it's also a technique that requires a tremendous amount of practice to get right. That's because inexperienced hands can do a lot of damage with a hammer and dolly, primarily by hitting the sheetmetal so hard, or so often, that the metal stretches, deforms, and warps.
Ken Sakamoto of Sunchaser Tools has spent years teaching hobbyists and professionals how to restore metal. He knows what he's talking about, having spent 17 years doing professional restoration work, followed by another 17 years selling bodywork tools. Among his students, contractors and carpenters are always the hardest to teach, he says, because whenever they have hammers in their hands, muscle memory demands they do one thing: swing like they're hitting a nail.
Instead, he teaches his students how to tap, not hit, the metal with the hammer while holding the dolly on the backside of the dent. The hammer taps push the raised metal down to conform to the shape of the dolly, a smoothing and leveling technique called hammer-on-dolly.
The weight of the hammer, the shape of the hammerhead, and the shape of the dolly are all carefully selected to fit the shape you want the metal in when the hammering is finished. For a rounded fender, use a dolly with a lot of radius on one side; for a flat quarter-panel or doorskin, pick a flatter dolly or the flatter side of the dolly. If you're hammering up against a seam and want to keep its straight line, use a hammer with a square, not round, head.
A second hammer/dolly technique is called hammer-off-dolly. As its name implies, the blows of the hammer aren't pushing the dent directly onto the dolly. The hammer lands around the dent's perimeter, while the dolly is in the dent's low center. By striking the metal, it and the dented area nearby move back close to original shape while minimizing the risk of stretching the metal in the dent.
If you're trying to hammer an area that's hard to reach with a dolly, such as the inside of a roof or the lower portion of a fender where access is only from the top, you can use a spoon, which is basically a flat dolly with a long handle attached to it. You may be able to straighten the metal in some of these areas simply by prying on it with the leverage of the spoon handle.
Shrinking from the Heat
If metal is hit hard enough-whether by accident or by over-enthusiastic, novice bodyworkers-it will stretch and deform. The best way to help the metal regain its original shape is through a process called heat shrinking. Heating the metal affects it on a molecular level, as the hot steel molecules can move back into their original contour with a little help from a body hammer. Cooling the heated area shrinks the metal close to its original thickness.
There are several ways to go about the heat-shrink process. The most common way is to heat a small portion (about the size of a dime) of the dent using an acetylene torch then quickly quench the area with a damp rag. The cherry-red glowing spot under the torch will be about 2,200 degrees F, so be very, very careful when near it. That amount of heat in a localized area can warp thin sheetmetal, so it's best not to use it on hoods or roofs.
If the dent is more of a crease than a dimple, don't hit it with a torch. All that will do is strengthen the crease, making it much harder to work out, says John Morrow of Johns Customz & Performance (JCP). Knock the crease out with a hammer, dolly, spoon, pick, or some other tool before heating the rest of the dented area.
A propane torch can be used instead of an acetylene torch to heat dented areas, Morrow says, though it will take longer to heat the metal, which can be a good thing if you need better heat control on thin metal. You can also heat small spots using an electrode tip on the electric nail-welding gun in a Uni-Spotter toolkit. As with the propane torch, the heat generated by the electricity won't come up as fast as with acetylene, but that may be more suitable to control the heat shrink for smaller dents or dents in thin metal.
Sunchaser's Sakamoto has developed his own method of panel repair called the Friction System. It utilizes a specialized tool-a heat-shrinking disc-mounted on a heavy-duty grinder. The wheel is run over a dent that's been roughed-out with a hammer and dolly, and it heats the metal to anywhere from 200 to 1,200 degrees, which has far less impact on the metal than an acetylene torch. According to Sakamoto, that's a good thing. Because you're making smaller changes to the metal's molecular structure, you have better control over the metal's movement and little effect on the undamaged metal surrounding the dent. Depending on the dent's severity, you may need to make 4, 5, 15 or 20 passes over it with the wheel. "Whatever it needs," Sakamoto says. The end result, though, is metal with such a fine, precise finish that many times no filler is needed prior to primer. That's why his system has been nicknamed the "no Bondo method" of sheetmetal repair.
(About once a month, Sakamoto teaches three-day, hands-on workshops that cover the use of his Friction System. Both Editor Kinnan and I have sampled it, and it's a fun and rewarding experiencewhether you're a novice or a longtime bodyworker.)
Body Filler Done Right
There's no shame in using filler on repaired metal. Even experienced bodyworkers will skim on a light coat in an effort to build a smooth foundation for paint. Filler has a bad reputation because it's sometimes used improperly or is used as a shortcut for repair methods better suited to the damage. Keep the filler layer to 11/48 inch or less-whether you're filling a small dent or finishing an area you've worked out-and you're using it the right way.
Most of the experts we spoke to said body filler should be applied only to clean, bare metal (that's been roughed with 36-grit paper) for best adhesion. Some will apply filler over epoxy primer, reasoning that since they are similar materials they will bond well. Others say the only bond that takes place between filler and epoxy primer is mechanical, not chemical; the bond is occurring only because the roughed-up epoxy primer has enough tooth to promote adhesion.
The photos here show JCP's Oraea Hunt applying and sanding filler on the same '65 Mustang rear panel you've seen elsewhere in this story. It was a cool day at JCP, but even with ambient temperatures hovering in the 55- to 60-degree range, Hunt had just a few minutes to spread the filler on the panel before it started to cure. Ten minutes after application, it was tacky to the touch; within half an hour, he was knocking it down with a grater.
Opinions vary as to how much hardener to use with the filler putty. Some say to use as little as 2 to 3 percent, but JCP's John Morrow uses what he calls the "bird turd" method: "Look close[ly] at bird droppings, and there's 80 percent white stuff and 20 percent other stuff. I use 80 percent putty to 20 percent hardener." Then again, a pro who's mixing filler looks like a top chef-there's no measuring, just a paddle full of putty and a squirt of hardener before mixing the batch. As we've said before, the good ones always make it look easy.
Nail It
What if you have a deep dent that needs to be roughed-out before heat shrinking, but you don't have the front-and-back access needed for a hammer and dolly? Get out the nail welder. These kits, sold under a number of different names, all work on the same principle: After removing all the paint from the dented area, you electrically spot-weld nails in several places in and across the dent and then grab 'em with the end of a slide hammer. Pulling on the nail with the hammer pulls out the dent. Once the dent is out, you cut or twist the nails off the fender, grind down the nubs, and skim a coat of filler over the repair.
There are "paintless" versions of this system, too, that glue rather than weld pullers to the dent. You don't need to remove the paint from the dent before gluing, and releasing agents are used on the glue after the dent is out to remove the pullers without marring the finish. Most of these systems, though, are geared toward thinner, late-model sheetmetal.
So what if your car's body woes go beyond a mere dent? What do you do about rusted panels or sheetmetal that's so badly damaged it can't be repaired? We'll get into those topics in our next installment.
