The cleaner the metal, the better your results will be. The reasons can be complex, but the improved results that clean metal surfaces provide are often visible and dramatic.
No one-size-fits-all prescription exists for metal cleaning processes, or how far to take them. Different jobs call for different mandates. Painting and plating are often the processes that are most intolerant of contamination. Welding is a bit more tolerant at first, but in the long run contamination can come back to bite you in the form of failed welds and cracks in the heat-affected zone (HAZ) adjacent to welds. Buffing and polishing seem to be less discriminating because they are also cleaning processes and often remove detritus that other cleaning processes have left behind.
Then, too, the standards for cleaning the metal used in your projects vary with the extent of the contamination. Here, as you might expect, rust is your toughest opponent. Although dirt, grease, paint, plating, and most other contaminants sit on the surface of metal or interlock mechanically with its surface nooks and crannies, rust is the result of a chemical reaction, and can burrow deep into metal.
In the case of some metals, such as diecast zinc, rust can form from impurities (lead, in the case of diecast zinc) in the metal. These impurities can be below the metal’s surface, so the rust can originate from the inside, out. All of that makes rust and other forms of corrosion much more difficult to remove than most other contaminants. This also makes it harder to prevent their recurrence once they have started to fester.
Now, I need to present a small but necessary bit of rust chemistry. This will be almost painless, and definitely not a chemistry lesson. I promise.
For rust to occur, three conditions are necessary: (1) A substrate must be present. That’s a surface to rust, and is a given. (2) A source of oxygen must exist as well as a medium to allow its transit to a site for rust to occur. Water, atmospheric moisture, and electrolytes, such as water contaminated with road salt, fit that bill and are ubiquitous. In the world of corrosion, you can think of electrolytes as “water on steroids.” (3) A circuit must exist to move the electrons that make the conversion of metals into their corroded oxide forms possible. In the case of rust in iron and steel, this is the conversion from Fe to Fe2O3; that is, from iron to iron oxide, or rust. (The chemistry is actually a bit more complicated than this and specific sequences are involved, but it roughly describes the conversion from iron to iron oxide.)
Because metals are, by their nature, electrically conductive, a circuit through them is another given.
So, the metal is a given and the circuit through it is another given, leaving only one foundation for rusting that you can manipulate: the presence of moisture or an electrolyte. The only practical way to stop rust is to deprive it of water, moisture, and, most important, electrolytes such as salty water. Okay, you have the basis for a plan. But, of course, there are several complications in implementing it.
The first is that the substrate has to be incredibly clean before you can even consider depriving it of moisture. Here’s why: If you leave any corrosion on a surface that you later coat, it contains moisture absorbed by the corrosion from the atmosphere. That moisture causes the substrate to rust further under your paint or other coating.
That’s critical because rust expands to roughly 17 times the volume of the iron or steel from which it forms. Other metallic oxides have similarly disastrous displacement rates.
Why do I call them disastrous? Because metal oxides exert enormous pressures as they expand to occupy their rightful space in nature. They literally push coatings, including paint, off substrates with the greatest of ease.
This panel surface was sandblasted and it then sat, unprotected, for a month. Rust specks and streaks have already appeared on it. If you try to prime and paint over them, you do so at great risk. The problem is what lies below the visible beachheads of rust.
In this drawing you see a little rust spot on top of a steel surface, penetrating deep into the metal’s granular structure. The penetration follows an electrical circuit along the metal’s grain boundaries. This makes it very difficult to eradicate the rust completely. Small rust specks can foretell big problems.
To make matters even worse, coatings, such as paint, have limits of elasticity and adhesion. If you push them far enough to stretch them beyond those limits, they fracture and detach from substrates. When they fracture, the fracture lines, literally cracks in the paint, tend to act like little capillary pumps that pull water and electrolytes in under the coatings where they have detached from surfaces. This adds moisture and/or electrolytes to the rust stew that is already brewing under the coating from the original bit of corrosion that was coated over. Then, the rust festers and expands some more, using the electrical circuit(s) through the metal’s granular structure that was established by the original rust that was left there under the coating.
The cracks in the finish get bigger and more cracks develop in the coating. More oxygen-bearing moisture enters through them. At this point, you have a corrosion cell, a veritable rust generator, under your coating. There is no way that this little mess can do anything but get worse, and worse, and worse.
Ospho is one of many available metal preps and conditioners. I’ve always had good results using it to protect unpainted steel surfaces until I can apply a finish to them. Ospho improves paint adhesion, too, by etching metal surfaces. Always rinse Ospho off with water before it dries.
This is how and why pinhead size rust spots under paint grow to dime, quarter, and then fifty cent size sores on their way to destroying whatever they are attacking. If this sounds like the scenario for a Saturday night horror movie, it might make a great one. My choice would be Bela Lugosi or Boris Karloff to play Count Corrosion.
Do not abandon hope. In this epic battle between good and evil, well, between rust and us, remember that we are free men and women with free will, intelligence, and resolve. We can analyze the situation and fight back with proven counter-measures. We can win. We will win. Here’s how.
With those two “givens,” metal and a circuit through it, and one “ubiquitous,” water, moisture, and/or electrolytes, you can have only one possible plan of action, and some strategies derived from it. Your plan must be to deprive the substrate of the things that give it the oxygen that causes it to produce rust, and other oxides in other metals. Foremost among those things is water and water containing electrolytes. Your plan is to deprive substrates of these evil rust nutrients and cut the rusting process off at the pass.
Implementing this plan means removing all rust from the substrate metal so that nothing is left there to hold moisture and to promote new rust under the coatings that you place over substrates. That is the first strategy. It can never be accomplished perfectly, but the cleaner you get your base metal, the less likely it is to rust after it is coated.
The second strategy is to either convert the clean substrate to something other than rust that is so chemically stable that oxygen is unlikely to displace it and then combine with the substrate metal to create new rust or other undesirable oxides. “How do I do that?” you ask. You can do that by converting it to more desirable oxides.
Every major paint company offers one or more reactive (etching or self-etching) primers.