The moment of truth has arrived. Your repaired, restored, or fabricated sheetmetal masterpiece is completed, and ready to be finished. That means that it will soon be hidden for nearly eternity, as far as you are concerned, under an opaque coating. Whatever type of finish is applied over your work—solvent-based paint, water-based paint, or powder-coated plastic—it will have an index of reflection that will reveal any defects in your work to a degree that the sanded metal that you now see never could. In fact, its coating will scream out any defects in the surface over which you have labored so long and so diligently. And no, painting it in flat, crackle, wrinkle, or hammertone black probably is not a practical workaround to avoid that shiny, painted reckoning.
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That is what makes this the moment of truth. It is when the quality of your results with skills like bumping, metal finishing, and filling are about to be put on display for all to see, and in the harshest possible way—under highly reflective paint. Then, it will have the opportunity to pass the first of the two pertinent tests applied to metal work: How good does it look, and how long will it look that good? It takes a while for the results of the second test to become known.
As you think about it, it is reassuring to remember that if you did everything right, made all the right decisions and moves, the quality of your metal work has built on itself at every stage. And it will pass that first test now, and the second test later.
Think of it this way: Great panel work involves the massive accumulation of favorable details. If there are 100 things—big to small—involved in a job that can be done between very badly and almost perfectly, and even perfectly, it is your mission to capture as many of those 100 things as is possible, as close to perfectly as is feasible. In almost all cases, this depends on the validity of the basic concept(s) that you used to approach your work, and on how many of its tasks you performed near flawlessly. If, from basic concept to small details, you did everything very well, you will have produced good-to-great work. Of course, small missteps can ruin a job, but it usually takes more than a single slipup to botch everything.
If you bumped or fabricated metal to very good approximations of final shapes, leaving little for filler to fill, this helps. If you attended to shrinks and stretches in the metal, as you formed it into correct shapes, you are a long way toward making its final format stable. If you avoided putting stress into the metal by forcing it harshly into position, and then welding it there, this helps some more. It is okay to slightly fine tune an edge with a small screwdriver, persuading it into perfect position for welding. However, if you made things line up with pry bars and 2-pound hammers, that will invite big problems, later.
If your metal finishing achieved perfect contours, as revealed by careful visual and tactile inspections, the odds in favor of it turning out between very well and perfectly are increased. If you made your filler application over metal that was completely cleansed of all traces of rust and other contamination, you have another considerable advantage.
If you used no filler, or used it very sparingly and in very minor thicknesses, you have a definite benefit. If you used lead filler, and were careful to neutralize all traces of flux and tallow residues as you went along, that is a big plus. If you used plastic filler and were careful to thoroughly mix its components, individually and with each other in correct proportions, that removes two more possibilities of failure at a later date.
Whichever kind of filler you used, if you were meticulously careful to sand out all file marks, and then to use abrasives with escalating grit numbers to erase all deep scratches left by previous and coarser grits, until you had produced a surface free of visible individual scratches, you have won the battle against sand-scratch swelling in the finish that will cover your work. If your last sanding was with an abrasive that left both metal and filler surfaces with good tooth for primer adhesion, you have headed off another potential issue that can plague metal work. If you cleaned your finished surface with solvent, and then blew off all loose debris with dry compressed air, you are another step toward perfect results.
If you protected your finished surface from corrosion with the one-two punch of a good metal conditioner, properly applied, followed quickly by priming and top coating, you have eliminated a whole class of failings and blemishes that can haunt metal work. If, instead of using metal conditioner, you went the other good-surface-protection route and quickly, after its completion, covered your work with an etching, waterproof primer, you have made a good move to exorcise the rust demons that can lurk on metal, under coatings.
As I said, if your basic approach was right, and you supported that by getting all, or an overwhelming majority, of the pertinent tasks and details of your work right, it should rank at or near the highest possible grade. It should look good and last long, and that is a substantial accomplishment, one in which you should take considerable pride.
The Danger from Behind
There is one more critical detail that accompanies the completion of metal work, before you paint it yourself or turn it over to the paint guys. It is the consideration of protecting the back side of your work from attack by moisture and the corrosion that inevitably follows prolonged contact with moisture. If your work fails at some future date, due to corrosion, it may be difficult to determine the source(s) of that failure. Perforation rust can originate on either side of a panel. When you do great metal work, protecting the back sides of your panels makes great sense. It won’t happen unless you do it or pay to have it done.
Your first chance to deal with potential corrosion is when you design a sheetmetal structure, or work on one that has obvious problems in this area. In the main, areas that trap water or, worse, water and dirt, and hold them against metal are places where corrosion is likely to begin and increase rapidly. Dirt tends to absorb water, and hold moisture against metal, long after just water would have left the scene due to evaporation, passing air, or by the momentum generated by vehicle movement.
One major corrective step that you can take is to avoid designing structures with moisture traps that hold water, and small openings that can use capillary action to draw moisture into narrow spaces. If you can imagine some of its features acquiring and/or trapping water when you look at a structure, you should try to prevent this from happening or to figure out a reliable way to cause them to drain.
At this point, you can very easily run into the law of unintended consequences, a variant of Murphy’s Law. My favorite formulation of Mr. Murphy’s dictum is the statement that “nature always sides with the hidden defect.” When dealing with corrosion prevention in sheetmetal structures, you have to identify all hidden defects and take countermeasures to overcome them. Then, you have to make sure that none of your countermeasures created new hidden defects along the way. If this happened, you have to take countermeasures against the flaws in your countermeasures.
For example, take the matter of sealing structures from the intrusion of moisture. Consider something like a headlight module system in failure, with fog coating the inside of its lens. It was designed to keep moisture out but the design not only let moisture get in, it actually trapped it there by preventing its exit. That is why it remains there. Another example is the seals on some bearings. They were designed to keep lubricant in, and dirt and moisture out. How many times have you seen such bearings hopelessly contaminated with what was supposed to be kept out and, due to their seal designs, all but impossible to relubricate? You have to be vigilant to avoid creating your own versions of these counterproductive situations. Providing for factors like drainage, venting, and cleaning access usually helps.
Consider the sealing systems that wipe against and seal car door windows. They are designed to keep water from seeping into doors, as windows are lowered, and rusting them out at vulnerable seams. Those vehicle door-window seals keep most of the water that could enter that way from leaking into doors. Unfortunately, they also help to seal in the moisture that does get past them and that arrives from other sources. Then, when the sun beats down on the outsides of these doors, with the potential to heat their cavities, evaporate the moisture there, and drive it out, those same wiper seals help to prevent this desirable outcome from occurring. The holes and vents in door bottoms that are designed to let water out often foul with dirt and debris, blocking its exit. Over the years, a few automotive body cavities have been fitted with vents designed to work on the Bernoulli principle—to use passing air to create a low-pressure situation to extract water from their insides. This works until some small bit of debris changes their configuration, and then becomes useless.
In many situations, keeping moisture away from metal is a very tricky proposition. I don’t care if they say that something difficult is like making water run uphill. Take my word for it, water will run up hill enthusiastically, when it senses that it can find something to rust by going there. Okay—I haven’t actually seen this happen.
Failed anti-corrosion designs and features are not hard to find. They are all around us. Just check out any junkyard.
Still, you shouldn’t be over-whelmed by the odds against taking perfectly effective action to prevent rust from mortally attacking your sheetmetal work someday. Just try to put that day off for a century or so. You have a big, last chance to make your final moves just before your project is painted. Later, you may have a few more chances to inspect, detect, and correct problems, but remember that corrosion looks for ways—24/7 and holidays—to destroy your work.
What You Can Do
The good news is that there are things that you can do. First and foremost, you can try to keep water from seeping in. This means designing seams and joints that are, and remain, tightly sealed against moisture. But that is not enough because moisture is pernicious; it can enter areas in airborne form, and condense into liquid. Even in airborne form, it can start and promote corrosion, without ever becoming a liquid.
Keeping water out is a noble aim, and should be pursued. But do so with the certain knowledge that your success with it will be partial at best. With that in mind, there is a second line of defense that you can pursue. If the structure that you wish to protect is one that you designed, you can eliminate obvious water traps like shelves and other enclosures. You may include drains for areas that might otherwise possibly trap water. You can vent potentially vulnerable areas, so that moisture has a chance to escape, or is extracted from them. You can make your vents and drains large enough to not plug up with debris.
The third line of defense is sealing and coating the unseen sides of your work. It starts with removing all traces of paint, rust, grease, and oil from the back sides of your metal work. After that, apply a good seam sealer, particularly to the back sides of lap and offset lap joints. Body caulk should be used in areas that are too wide open for seam sealers to fully close them. Back side metal should be coated with a waterproof and very dense (in the molecular sense) coating like moisture-cure urethane. Applied to steel, moisture-cure urethane aggressively draws water molecules from both of its surfaces, the one exposed to the environment and the one facing metal. That means that it removes all moisture from what is under it—a considerable advantage in fighting rust.
The molecular structure of a moisture-cure urethane coating is so dense that even the relatively small, frenetically active H2O molecule has great difficulty penetrating it. While moisture-cure urethane’s surface characteristics, and its lack of robust resistance to ultraviolet light, make it inappropriate for finishing vehicle topsides, it is ideal for protecting their undersides. It clings to properly prepared metal tenaciously, blocks the transit of moisture, and remains resilient for decades after it is applied. This means that it is unlikely to crack under the assault of flying gravel and other things kicked up by vehicle wheels.
After you have completely sealed the back side of your metal work, go for some extra insurance. Remember, anyone who wears both a belt and suspenders is unlikely ever to suffer the embarrassment of lower wardrobe failure. The extra insurance is to coat the painted underside of your work with a flexible and resilient corrosion-protection agent. This could be a hot-sprayed, paraffin-based rustproofer that contains anti-corrosion additives, or a rubberized undercoating.
Finally, you should check these measures, after their applications have settled, to make sure that there are no gaps or other flaws in them. For example, be sure that you applied the undercoating consistently. And be sure that it didn’t flow downward, potentially creating gaps, and possibly clogging or blocking critical drains and/or vents.
With all of those measures, you may still fail to provide your work with perfect, or even adequate, protection against rust. This is true for both custom work and for modifications that you may make to improve the corrosion resistance in existing vehicle designs. Sadly, in the end, rust always triumphs. The real issue is what we can do to deny it that victory for the longest possible time. If we gain some valuable time against the rust enemy, that alone is worth celebrating.
What I have written on this topic indicates how tricky rust prevention problems can be, and how carefully you have to develop and deploy your countermeasures to them. Here is one last example: In early 1977, I took delivery of a new Honda Accord, the car that arguably established Honda as a major player in North America. Immediately after delivery, I took the car to have it undercoated by a friend who operated a Ziebart rustproofing establishment.
When I returned to pick it up, my friend said, “Matt, I want to show you something.” He produced a memo from Ziebart’s home office in Connecticut, noting that their franchises were required to advise their customers that the standard Ziebart warranty would not apply to the front fenders of Honda Accords sold in the United States and Canada. In fact, Ziebart would provide no warranty at all on the front fenders of these cars.
The memo went on to state that the design of these fenders made it impossible to protect them with the Ziebart rustproofing process. As it happened, my friend had another Accord in for rustproofing that day. He walked me over to it, drilled an access hole in its right front fender door-jamb-facing surface, and inserted his undercoating wand through the hole. He told me to watch the metal on the top of the fender, near the cowl and hood, as he shot undercoating at it. Then, he triggered his spray wand and withdrew it through the drilled hole, spraying high-pressure, hot paraffin under-coating from its nozzle at the fender top’s underside. As I watched, I saw the metal in the fender’s top deflect slightly, from the pressure of the undercoating hitting its other side.
“Wow,” was all that I could say. My friend agreed, and noted that he had never seen anything like it. Honda’s die-stamping process had drawn the fender top metal so thin that it could be moved by an under-coating spray.
Worse, that thin metal flexed so much during the car’s operation that Ziebart had determined that any coating applied to it would tend to fracture in short order. Ziebart further concluded that once the paint fractured, it would pull in moisture and would quickly rust. The rust would expand and release the coating, leaving the thin steel bare and wide open for corrosion.
It got worse. Because the air under the fender tops exchanged with the air in the engine compartment, moisture would condense from there, as that warm area cooled when the engine was shut down. Cyclical engine-compartment cooling and condensing moisture is a natural occurrence in many climates. This predictably happens when an engine is shut down at the end of a day.
Ziebart was right. Its corrosion engineers had foreseen a situation that Honda’s designers and engineers had not. Within a few years, almost every early Accord that one saw in northern climates was either rusting about its fender tops, or sporting replaced fenders. Honda even offered replacement fenders for little or nothing, if the customer would pay for installation and painting. This offer was kept open for a long time after the original warranties on these cars had expired.
I tell this story to make several points. Honda had very good body engineering in that period, but failed to foresee this problem with the Accord. Ignorance of the North American climate and of salt use here probably contributed to creating this design flaw. Besides, these kinds of failures are usually far from obvious. In this case, the actual problem involved a sequence of factors and events that worked together to produce a perfect corrosion storm. Corrosion failure is rarely attributable to a single cause or fault. Later Accord fenders looked very similar to the early ones, but did not suffer any unusual or premature corrosion problems in their fender top areas. These problems can usually be solved fairly easily, once their exact causes are known.
The best action that you can take to prevent back side-initiated corrosion from ruining your work is to consider the possibility that it can happen, in every action that you take. Think about it when you decide on joint design, or when you seal the backs of your joints. Consider it when you paint your work, or instruct someone else on how you want it painted. In sum, be aware of any factor, series of factors, or combination of factors that might result in a successful corrosive attack on your work. Then take the best countermeasures that you can.
Written by Matt Joseph and Posted with Permission of CarTechBooks