Keep in mind that in most cases cleaning a few similar items does not demand the very high efficiency that cleaning a large number of similar items does.
The intricacy, fragility, and consistency of the object you are cleaning also make a big difference in your choice of process. Attacking an engine block with solvent and scrapers works, but so does the automated cleaning system of oven baking, shot peening, and jet washing used by many automotive machine shops. The latter, however, is simply more thorough, faster, and just plain better than poking at an engine block with solvents and scrapers. It also has the important added advantage of relieving stress in the block’s metal. Of course, there are many good approaches between those two extremes.
The kind of metal and its condition are central to your choice of the most effective cleaning approach. Ferrous metals, including steel and iron, usually take quite a beating if they are not compromised by severe corrosion or structural stress damage. They can be subjected to physically and chemically rough processes and survive intact. However, thin sections of steel, and particularly metals, including copper, aluminum, brass, and bronze, tend to be more delicate and do not take much cleaning abuse. Badly corroded steel (also known as French lace) must be cleaned very delicately to avoid damaging it beyond the possibility of repair. The same is true of badly corroded aluminum and brass, except that they often tend to be even more brittle and fragile than damaged steel.
An item such as a thin steel, brass, or bronze nameplate that is in good condition is inherently delicate and must be treated as such. Hand brushing with a soft brass or stainless-bristle wire brush, chemical cleaning, ultrasonic cleansing, or soda blasting may be the ticket to get such a piece clean enough to repair and/or refinish properly. However, blasting with a peening media, including glass bead, is likely to stretch and warp it, while attacking it with an aggressive abrasive blast media such as aluminum oxide or silicon carbide would probably be unnecessarily violent and might warp or cut through the item. Blasting with an agricultural media, such as pulverized walnut shells, peach pits, or corncobs, is slow, but mild and very useful for dealing with small, fragile parts. Tumbling and vibratory approaches are also possible in this case.
Coatings over clean metal are the first line of defense against corrosion. However, when painted surfaces are as compromised as the one shown here, the coating does no good and can accelerate the rusting process by creating protected cells under which rust propagates.
It is critical to keep in mind that each individual part, or batch of parts, has to be considered separately with regard to many factors (that include size, type, material, and condition) when you choose a cleaning process, or processes. Many situations benefit from the application of multiple cleaning processes. Often these processes are staged to enhance a part’s safety and the cleaning result.
What you are removing from metal can be as important as the size, type, configuration, and condition of the metal itself. Contaminants that sit on the surface of metal are easier to deal with than those that penetrate into its intricacies and pores. With the possible exception of extreme stress, corrosion is the foremost enemy of metals, and is far more common than stress damage. It is a product of metals’ natural degradation, involving the tendency to combine with oxygen to form oxides. These oxides are capable of penetrating deep into and below metal surfaces and into their granular structures. In the case of steel it’s called degradation “rust.”
Welding over contamination causes defects. The top of this weld was made in cleaned metal, while its bottom was welded through dirty, rusted metal. The dirty weld has visible floating contamination in it. Further-more, amperage variations caused by welding over contamination have bulged the surrounding metal.
Because buffing is cleaning, buffed parts such as this nameplate may seem to be safe from contamination, except buffing wax. However, if you get fingerprints on them before you can coat and protect them, those fingerprints may develop under the coating (like the ones on an FBI wanted poster).
This weld was allowed to age without protective coating. It shows rusting in the heat-affected zone (HAZ). That is the area from the weld out that absorbs enough heat to affect the surrounding steel. Note that the visible rusting in the HAZ is worse than that on the weld itself.
In a more general sense, corrosion is an example of the process of entropy, as described by the second law of thermodynamics. It states simply that everything in the universe tends toward a lower state of energy, sort of like a clock running down. You probably never thought of the rust that keeps trying to devour your classic Ford, Chevy, or Mopar in that sense.
Corrosion is not the exclusive disease of steel and iron. Other metals also oxidize, but without showing telltale signs of corrosion. It comes in the form of the red flakes, pits, and powders on the surfaces, the things that you associate with rust. Aluminum oxide is the product of the corrosion of aluminum and its alloys, and is white/gray in color with a powdery texture. It is every bit as deadly to aluminum alloys as rust is to ferrous metals. Copper produces a green oxidation product. Other metals show other characteristic signs of oxidation. A few metals and engineered platings do not corrode under normal conditions, but these are uncommon in the general run of automotive metal surfaces. Some of them are used to plate ferrous metals to protect them. Cadmium and copper/nickel/chromium plating is commonly used for this purpose and to highlight some trim parts.
Non-magnetic stainless steel and hexavalent and trivalent plating of metallic compositions show notable resistance to corrosion. However, these are way off the beaten track of the metal surfaces that you are likely to find on automotive parts and panels.
Corrosion may be confined to just the surface of a part or panel. Superficial rust is relatively easy to remove and to prevent from recurring. Deep, pitting rust is another matter. It is difficult to eradicate it and prevent its recurrence. That’s because metals have granular structures. Rust and other corrosion tend to form along the lines of grain boundaries. Once rust travels below the surface and deep into metal it becomes much more difficult to deal with, but not impossible.
Although rust is the contaminant most likely to burrow deep into metal’s pores and granular structure, it is not the only contaminant that must be eradicated to achieve clean metal surfaces. Paint coatings must often, but not always, be completely removed to refinish auto body panels and parts. Grease, oil, and, silicone have to be removed from metal surfaces for painting, welding, soldering, and plating to adhere properly to them. If you try to apply paint over these impurities it fisheyes, at best. If a finish manages to cover them at all, it fails to gain proper adhesion and fails.
Silicone is particularly irksome in this regard because you cannot see it and it can be difficult to remove. Even minute amounts of silicone combat paints’ surface tension and ability to cover. Solder does not wet properly over oil, grease, and silicone, so proper tinning of surfaces becomes impossible.
Welding over contamination is in a class by itself. Okay, some welding electrodes are labeled for use on dirty and/or corroded metal. They may be capable of wetting and beading on such surfaces, but that only wins a battle, not the war. The more that you learn about hydrogen embitterment and other crack causing phenomena in weldments, the more implicated are impurities that release hydrogen and/or sulfur. Welding dirty metal is an invitation for later cracking caused by hydrogen embitterment or sulfur contamination,