The only sure way to stop the rust in this decklid seam is to remove the skin and replace it, or to repair its damaged areas with new metal. Cleaning the area thoroughly and applying moisture-cure urethane paint or seam sealer to the exposed outside joint may work temporarily.
Happily, areas such as panel edge seams are exceptional in the difficulties they present in cleaning and sealing. Most surfaces on cars are easier to inspect and clean. Take, for example, the outer surface of an auto body panel, including a fender or a hood. The metal in them is almost completely accessible, either mounted on a car or removed from it.
How clean does a fender or hood have to be before you can weld or paint it? Here’s a hint before you try to answer that one. (“Perfectly” isn’t the right answer, because that is an unattainable state in the real world of cleaning.) In fact, that fender or hood surface doesn’t have to be “laboratory” or “metallurgically” clean to weld it or to have it hold paint reliably. But it will likely need to be as clean as you can get it for those purposes.
Seam-sealing products such as these are designed to seal crimped seams. The two on the left from POR 15 are intended for application over rusty metal. They leach moisture out of rusty seams and protect them with a dense, waterproof coating.
Solvent parts baths such as this one are mainstays for cleaning mechanical parts. They perform well in that role if sediment is removed from them regularly and their solvents are replaced when they become contaminated. They should never be the final step in cleaning for painting.
How clean something looks and how clean it is are usually critically different. Paint and rust are at the easy end of the scale of contaminants that you have to remove from metal. You can pretty much see where they are and when they have been mostly removed. Or can you? If you carry your inspection to a microscopic level you often see remnants of things that you thought you had thoroughly expunged.
The 3D microscope in the center lets you look at metal surfaces far beyond what you need to inspect for decontamination. However, the small, handheld, illuminated pocket microscope (left) and the illuminated magnifying glass (right) are great tools for inspecting metal surfaces for cleanliness.
Then there are contaminants like oil, grease, and silicon. They tend to be invisible to the unaided eye. Yet any of them in small concentration can cause paint to fisheye and/or to lack adhesion. Even the fingerprint of anyone guilty of touching unprotected metal deposits enough body oils and moisture to become a potential cause of painting defects.
Some contamination does not attack surfaces from the outside; it lurks beneath them. For example, lead impurity in some diecast zinc items, such as trim and door handles, can corrode these parts from the inside out. In this case, corrosion cells start beneath the surface and work out. Removing all traces of corrosive contaminants from these parts’ surfaces and subsurfaces is challenging in the extreme.
The truth is that while you can never get anything absolutely clean, it doesn’t hurt to try, even if you never achieve it. At best you will dilute contamination from the surfaces of metal to levels that are not a problem for your next steps: welding, painting, plating, etc. This doesn’t sound very elegant or hopeful, but it is very serviceable. Get metal surfaces as clean as you can, but realize that there is always something left lurking on them. If you dilute that something sufficiently, you have used a tactic that will probably win your war.
In the case of rust, you may leave some of it behind, but if you convert the metal surface to a stable form that is not rust, you have again used a tactic that gives you an excellent chance of winning your battle with corrosion.
Metal wiping solutions such as DuPont Prep-Sol do an excellent job of removing oil, grease, and particularly, silicone from metal surfaces. Less expensive fluids including trichlorethane-based brake cleaners and enamel reducers also remove oil and grease but do not remove silicone residues as completely as do wiping solutions from paint companies.
Use a metal wash to remove oils and silicone before you apply a metal converter solution or etching primer; it will likely save your project from defeat by some insidious and largely invisible enemy of paint adhesion.
Barrier coatings such as moisturecure urethane paint paste or caulk leach moisture out of a substrate and are great weapons in your battle.
Employing a slow-dry enamel to encapsulate and isolate rust is also a serviceable strategy against corrosion.
None of these weapons or tactics results in outright, automatic, or irreversible wins, but if they allow you to take enough of the rust enemy prisoner, you will likely win the war with corrosion for a very long time.
BASIC APPROACHES TO CLEANING METAL
“Scratch, scratch.” That’s the sound of cleaning metal. Okay, you can’t always hear it, and it doesn’t always sound just like that, but abrasion is the key; it is, literally, the only way that you can clean metal.
“Sure,” you say, “that covers the old routines: wire brush, sanding disk, abrasive blasting, and the like, but what about laser, ultrasonic, and chemical cleaning, and other approaches that don’t seem to scratch anything?”
In truth, all cleaning methods involve abrasion at some level, mighty or small. Once you get beyond the obvious abrasive methods, the abrasion tends to be subtler, but it’s still abrasion. Approaches such as sanding, abrasive blasting, buffing, and polishing may keep the scratching at sub-visible levels, but they don’t eliminate it.
I’m not saying that the mean old iceberg was just trying to clean the poor Titanic’s hull, but you could look at it that way.
Take these examples: solvent and ultrasonic cleaning. In the first case, the scrubbing is done by the natural action of solvent molecules against surfaces and the things that contaminate them.
In the case of ultrasonic cleansing, the machines that do it tend to emit a slight, audible hum and hiss, but that isn’t the part of the sound spectrum that is actually doing the cleaning. In an ultrasonic cleaning tank, shock waves that are invisible to the eye and soundless to the ear are imploding water-based cleaning agents against the tank and its contents, and cavitating those surfaces that are in contact with them and the liquid cleaning agent.
Behold the most basic of automotive cleaning processes: good old solvent and a bristle brush. What it lacks in high tech shock, awe, and pizzazz, it makes up by being inexpensive, effective, and dependable. The solvent loosens and dissolves dirt and grime and the bristle brush pushes it off surfaces.