Most items to be cleaned for powder coating are made of iron, steel, or aluminum. A variety of other metals are also used, including brass and copper.
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Different rules apply when cleaning various metals. Some of the stronger chemical cleaners destroy aluminum-based alloys, for example, so it pays to know which metal you’re cleaning and which product is best for it. Because each part is different, you may need to adjust your process. Soon you will be able to look at a part and tell what tools and sup-plies in your arsenal will get it perfectly clean.
The projects in this chapter came from parts bins and other sources to be processed for powder coating. They are representative examples of what you might undertake.
Iron and Steel
The upside to cleaning iron and steel is that very few products attack these metals with the ferocity that caustics attack aluminum; you’re certainly unlikely to find any of these caustics in your garage!
The downside of iron and steel is that rust is prevalent. As mentioned previously, a couple of basic chemical products work on rust: phosphoric acid–based naval jelly (also called etching solution) and urea hydrochloride–based rust dissolver.
Cleaning Deep Rust
For this sample deep rust project, I selected the bottom cap from a welding tank. Because water often condenses in these tanks, the bottoms can become quite rusty. Because this part formed a natural bowl, both chemical and mechanical processes could be used to strip it.
I started by chipping away loose rust with a scraper, and then followed up with a good soak in East-wood Fast Etch to let the acid work on the rust. When the acid was neutralized, I used an auto body grinder with a wire brush cup attachment to remove the remaining loose material.
Cleaning a Steel Part
For a steel cleaning project, I chose the bottom portion of an old welding tank. This tank had been cut into pieces to create outdoor wind chimes and would benefit greatly from a protective powder coating.
Because of the high-quality paints used on welding tanks, chemical paint removers had no effect. The best cleaning process for this part was an auto body grinder with a wire brush cup, followed by a wipe-down with Eastwood PRE Painting Prep.
Cleaning a Cast-Iron or Steel Part
Cast iron rusts differently than other forms of iron or steel and responds well to chemical-based rust removers. For this project, I picked an old brake disc. Because these are subjected to daily heat cycles, they tend to rust everywhere that they are not constantly scrubbed by the brake pads.
The solution was Eastwood Fast Etch, which removed the rust, and then a good sanding and wire brushing. Brake pads leave a layer of residue, so if you are planning to coat a brake disc, be sure to use the sandpaper step.
It bears mentioning that because brake discs routinely become hot, it’s not appropriate to coat a disc that will actually be used to stop a car. But if you’re making sculpture, go ahead.
When you remove the grease or dirt from an aluminum part, remember that aluminum and its alloys are lighter and softer than steel, so be careful not to abuse them with tools such as an angle grinder and wire wheel brushes; they dig right in and chew up the metal.
Also, aluminum and pot metal are very sensitive to caustics; they eat the metal like pouring paint thinner into a Styrofoam cup. Be sure to use methylene chloride cleaners on aluminum parts. Also, make it a habit to read the label on any chemicals you might use on aluminum; the packaging states whether they can be used on specific metals.
Cleaning an Aluminum Part
For an aluminum cleaning project, I chose a used bell housing from a Corvette transmission. It was caked with oily gunk made of leaked and cooked engine oil mixed with dust from the clutch disc.
I first applied oven cleaner, which loosened the material somewhat, but didn’t come close to removing all of the accumulated gunk.
Next I soaked the gunky areas in a detergent-based parts washer, which helped a bit more, but still left it far from finished. With the investment of time and effort balanced against the relatively low (about $20) cost to have this part professionally cleaned, I elected to send this one out for cleaning.
You can easily powder coat any metal that conducts electricity, which opens the door to coating copper, bronze, pewter, tin, and a wide variety of other materials, including those that have been plated with cadmium, nickel, chrome, or even silver or gold.
Cleaning a Brass or Copper Part
For cleaning brass, you need a chemical that removes the oxidized metal and impurities that form tarnish. Products such as Tarn-X are readily available at the grocery store, but Eastwood’s Rust Dissolver works perfectly on copper. Since both are a mixture of acid and urea, they work essentially the same way.
Simply coat the copper or dip it in the tarnish or rust remover for a few seconds; the process on copper is amazingly fast. You may still need to scrub to remove foreign materials such as paint or gunk from the copper, but this is easily done with a Scotch-Brite pad or steel wool.
Cleaning a Plated Item
When cleaning plated objects, be gentle, and don’t use destructive tools such as a wire wheel. You can use standard tarnish remover from the grocery store, or just use baking soda or borax with boiling water.
If some of the chrome plating is flaking off, you have to take it all off. However, if the plating is in good shape, you can coat it.
Because these kinds of metals are generally used for valuable parts, this is another great use for clear powders. Just be careful to preserve the shine and the color of the underlying metal through the cleaning process to get great results.
Repairing Holes, Cracks and Divots Before Coating
Occasionally you want to powder coat a part that needs some metal repair before you begin. This is especially true for automotive restoration projects, and is likely to happen more often than you might expect because powder coat is an excellent way to hide previous metal repairs.
Getting the right supplies for your project is critical. You may have seen MacGyver on TV making a weld with a pair of jumper cables and a quarter, but that’s really not a recommended technique. The good news is that you’re not likely to need everything you see in a welding shop.
A 120-volt wire-feed welder (readily available at discount tool stores such as Harbor Freight) and some general-purpose solder/filler rod and a handheld propane torch gets you through almost all steel projects.
Working with aluminum is trickier, and requires MIG or TIG welding; this is best left to professionals if you don’t already have the equipment.
By the way, if you need to weld cast iron, a stick welder can be rented at most welding shops, and the correct welding rod is readily available.
If you plan to use a wire-feed welder on steel, you have two basic choices for welding wire: flux-core and solid metal. Flux-core wire includes its own welding flux, which is a material that burns and creates the air-free area in the vicinity of the weld. The flux also forms a crust over the surface of the weld that you need to chip or brush off as the weld cools. If you try to weld over the crust, some of it becomes trapped as a bubble in the weld, which reduces weld strength and looks ugly.
The benefit of flux-core wire is that you don’t have to deal with a bottle of gas. You just plug in your welder and start working. The down-side is that flux-core wire requires AC operation and produces a lot of smoke and splatter. Flux-core is also not recommended for welding thin (20 gauge or less) materials.
With a small bottle of MIG gas (generally argon or a mixture of inert gases) you can work using DC power and get a much smoother weld, but then the whole assembly is more expensive. Common flux-core wire choices for automotive work include Lincoln NR-211 or any wire that meets the American Welding Society (AWS) E71T-GS standard.
Welder Power Level
For a basic flux-core wire-feed welder, select a power level appropriate to the thickness of the material. Your welder may have switches or a dial to control the power, but the adjustment process is the same: Start at a lower power than you think you need and run a few test beads. Check for adequate penetration by turning the piece over; when you can see discoloration from the weld on the backside of the metal, you’re working most of the way through the metal.
Wire speed is a function of the amount of power you select. If your welder uses a pair of switches to control the welding power, you have four power settings from which to choose. In general, the more power you use, the faster the wire speed.
Set your welder to its lowest power setting and set your wire speed at about 30 percent of its capacity, then try a few practice welds. Adjust the wire speed until you get a smooth flow into the weld.
If the wire speed is too fast, the wire pushes the welding gun back, or the wire breaks off and leaves short lengths sticking out of the weld. If the wire speed is too slow, the arc skips and sputters and the weld is uneven.
Adjust the wire speed until you get a good, smooth arc and weld bead.
Filling Holes Using Rosette Welding
The most common welding repair you’re likely to perform before powder coating a part is to weld up a hole that someone previously drilled in a part. Welders use a rosette (or “plug”) weld to fill small holes, up to about 1/2 inch in diameter. You simply weld in a circular motion to deposit filler rod around the perimeter of the hole until it is closed up.
The name “rosette” describes the weld well; when you do it right, it looks a bit like a flower.
Smoothing Welds with a Hammer
Another technique that comes in handy when repairing parts before coating is hammer welding. With this technique, you simply hammer down the “scar” of a weld bead while the weld is still hot and the metal is malleable. Use a bodywork hammer and dolly for this, but remember that you will still likely have to file or grind the surface smooth. Be sure you don’t pound the part out of shape.
Filling and Smoothing with Solder
If you have a part with a divot or deep pits, or that simply needs to be smoothed over before coating, a really convenient method is to fill the area with solder. In the past, pure lead was the material of choice, but lead and lead fumes are toxic and should be avoided if possible. East-wood makes a lead-free solder specifically designed for smoothing over divots. Its main use is for automotive bodywork, but it works well on smaller metal parts that you want to powder coat.
Eastwood’s solder melts with the heat of a handheld plumber’s propane torch, but stays strong at the temperatures used to cure powder coat. The soldering rod starts to become malleable at 428 degrees F, and works best at 535 to 660 degrees F. Because all powders you will be using cure at lower temperatures, this solder is perfect for part repairs and smoothing. The solder is always softer than steel or aluminum, and you can smooth it over with a file or sandpaper.
Be careful with grinders, however; they eat this stuff very quickly. In addition, safety is always paramount. Even if it’s lead-free, always wear a respirator when working with solder.
You may have used a soldering iron on electrical components; this process is fundamentally the same: You melt a soft metal filler material (usually an alloy of lead, tin, and other metals) and use it to attach a wire or transistor/resistor/diode to a fixed point. The molten soldering metal flows and sticks to both parts and when it freezes, it has enough grip to hold the wire or component in place under reasonable stress.
The same principle applies to repairing metal parts: You melt the solder and it fills the gaps or pits in the metal, and it stays in place when the metal cools.
Here are the basic steps for using solder:
Make sure the part is completely clean.
Fire up your plumber’s handheld torch (available at any hardware store) and hold the rod up to the area on the part that you wish to fill.
Heat the end of the rod until the solder melts and then spread the molten material around with a metal spatula until it covers the area.
Reheat as necessary while you work. The solder hardens quickly as it cools.
Solder is not nearly as hard as steel or even aluminum, so once it is cool, you can smooth the solder to the level of the surrounding part with a hand file and then sand to a smooth surface suitable for powder coating.
Be sure to clean the part after you finish the repair.
Written by Jeffery Zurschmeide and Posted with Permission of CarTechBooks