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I'm building an F-100 truck, and the fenders have several cracked areas around the wheelwells. Some of the repaired areas have a length of rebar welded inside the fender lip. My first concern is that, considering the age of this truck, the metal has started to crystallize, and this is what's causing the fenders to crack. Is there any way to reverse this process?
Another concern is what to do with the fender edges that are reinforced with rebar-there are some new cracks that are forming at the ends of these areas. I plan to drive this truck for a long time when it's finished, and I want to be sure that I fix these problems in the correct way so I don't have any problems in the future!
Via the Internet
Crack repair is part of just about every classic truck-building project-usually, the older the truck, the bigger the problem. Let's look at the issues you've raised.
The term "crystallized" is often misused when talking about metals. All metals are made from crystals (also called grains), so this is not something that happens over time; it's an inherent property of all metals. There is something that happens over time, however, to parts that are flexed continually-the metal becomes brittle, or work hardened (engineers call this strain hardening). Steel resists the effects of work hardening pretty well-a steel part can go through millions and millions of cycles without any problem (by contrast, brass and aluminum work harden much more quickly than steel). But each flexing of sheetmetal does contribute to hardening in a minute but cumulative manner, and fenders on trucks have usually taken quite a beating, especially if they were heavily loaded and driven over rough roads (as is common for work trucks).
The saving grace of this is that most likely your truck will have a pretty soft life, compared to its earlier incarnation, so if you do a proper repair on the existing damage, the odds are good that the repairs will last for dozens of years.
As long as the metal is full thickness, a crack can be welded back together, and if the weld has full penetration, it should make a lasting repair. TIG, MIG, and oxyacetylene welding are all good processes for repairs of this type. Unfortunately, it is not uncommon for the metal at the crack to be thinned, especially if someone has repaired the area previously and used a heavy hand with a file or grinder before or after the weld was made. If the metal at the crack is substantially thinned, I'd suggest cutting away the entire area that is questionable, and replacing it with new, full-thickness metal.
I've seen many, many fenders that have edges reinforced with steel bar, or as in your case, rebar. There is no question that welding bar to the fender edge reinforces it. The problem is that you're likely to get new cracks at the ends of the bar, just as you have reported. The problem here is that the reinforced section is very stiff, but as the fender vibrates, the flexing concentrates where the reinforcement ends abruptly against the single-thickness area of the fender edge.
One solution is to run the reinforcing bar along the entire edge. I've seen people use bar stock in very large sizes for this purpose, which is overkill; 1/4- or 3/8-inch diameter is plenty. And it's better not to weld this bar continuously, since it's likely that this much welding will cause serious distortion. You can skip-weld, with 1/2-inch-long welds spaced about 1-inch apart. Another approach is to weld in a shorter section of bar, but to grind a long taper into the ends of the bar so there isn't an abrupt transition between the reinforced area and the single-thickness metal. Another approach (which I prefer over using bar stock) is to form a strip of sheetmetal to fit inside the fender lip, and then skip-weld this to the fender edge. This makes a low-profile reinforcement that I think looks better when you look at the inside of the fender. I don't recommend welding the edge of this strip that rests against the fender away from the edge-this will surely cause distortion that will be very difficult to fix. It is a good idea, after all the welding is done, to apply seam sealer to the inner edge of the reinforcement-this will prevent moisture from creeping in behind the strip, which could cause rusting later on.
I'd recommend cutting out the rebar and using small sheetmetal reinforcements in the areas where the cracked areas extend to the fender edge. When new cracks form, they often start right next to where the old crack was repaired, so if you give this area a little reinforcement, it will often prevent further problems from occurring for a very long time. Of course, reinforcing the entire fender edge with round bar will make it even stronger, but rebar is a pretty gnarly form of steel, and I don't recommend it for this application!