When you set out to create any kind of finish, you learn that your prep work is the most important factor in getting good results. When it comes to powder coating, extreme cleanliness is absolutely critical. It’s easy to understand: If there’s any oil or dust on the item you’re trying to coat, that material is trapped between the powder and the underlying metal, and sooner or later (usually sooner) it separates from the metal and causes the finish to bubble and flake away.
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The challenge with cleaning is to get everything absolutely clean without turning your workshop or garage into a toxic waste dump.
Water and detergent-based parts cleaners are generally better to use in home shops than solvents. Water-based products create less hazardous waste, and they can deliver effective cleaning performance. Water-based products also eliminate a big fire hazard in your shop. Some gunk, however, responds better to solvents than detergents.
In addition to grime (grease and dirt), you may also need to remove rust and old paint. No one solvent or process is best for all three offenders. If you are using solvents only, consider rotating your parts through different solutions. For example, use a parts dip to remove grease and dirt, then a paint remover, and finally a rust remover. Repeat if necessary. Rinse and dry the parts between solutions so you don’t contaminate one solvent with another or cause unwanted or even dangerous chemical reactions. If you are planning to media blast, cleaning the parts with solvent first speeds up the process.
Mechanical cleaning also has its place, as discussed in Chapter 4. When you have a lot of material to remove, you may want to start with one of those mechanical methods, such as a scraper, wire brush, sandpaper, media blasting with soda, sand, walnut shells, or glass beads. Then you may want to finish with a wet cleaner such as one of the solvents discussed in this chapter.
Chemical methods of cleaning metal vary widely, and the best ones to use vary based on the type of metal you’re cleaning and the type of gunk you’re cleaning off the metal.
Remember that any chemical intended to clean metal can be very toxic, and can “burn” your skin. Be sure to have several sets of chemical-resistant gloves near each parts washer. You don’t want this stuff on your skin. Always read the labels and follow the safety precautions that manufacturers recommend; add a good helping of your own common sense, too.
Cleaning Agents and Solutions
You can start with a bucket and soapy water, or a large can of solvent such as B12 Chemtool or mineral spirits. For a bit more money, you can buy a benchtop or freestanding parts washer that uses either water or solvent-based fluids. Many restorers use gasoline, biodiesel, or kerosene instead of the more expensive solvents in those cleaners.
If you plan to clean a lot of parts, and you have a sewer connection in your garage or shop, you can usually obtain a working dishwasher at very low cost and install it in your shop. Take out the top rack and you can fit some very large pieces into it and wash them with detergent and steaming hot water. This works very well.
Each of these options has some trade-offs, and that’s why many shops have more than one type of cleaning station. Many solvents dissolve or substantially weaken rubber parts, including the diaphragms in mechanical fuel pumps. Room-temperature water-based cleansers are less effective on grease and oil-based gunk, even with soap or detergent additives. Water can also pose a rust problem on some surfaces.
In general, you can do a good job with a small benchtop solvent-based parts washer and a bucket with some Simple Green or Oil Eater detergent and hot tap water. If you think you might need more, you can get a couple different benchtop units and fill them with different materials. Be sure to label each one!
Water and Detergent
The safest cleaners you can use are detergent and water. Water alone doesn’t strip dirt or many other impurities on the surface of a project, but detergents are effective because they dissolve oils and greases and hold dirt in suspension while you wash. Note that soaps can also be used, but they often leave behind more residue than detergents.
Hot or warm water works better than cold water simply because so much of the gunk that needs to be removed from automotive parts is grease based. The more you can melt the grease and oily stuff, the easier it comes off. Pressurized water is also an effective way to get rid of buildup and even paint. So it stands to reason that hot, pressurized water with some detergent in it is the most effective way to use detergent-based cleaners. If you have a hose bib that delivers hot water, connect it to the pressure washer rather than connecting to it a cold-water source.
You can generally rent a pressure washer with a kerosene-fired preheater for the water and a siphon to pull in some detergent, but these are mainly used for big jobs such as cleaning the underside of a car or an entire engine. Still, if you have enough material to clean, it may be worth the rental fee from time to time.
One of the more effective cleaners you can pick up in any grocery store is ordinary oven-cleaning foam. You simply spray it on and let it work, then wash it off with water. This is particularly good at dissolving gunk that has been baked onto your parts. But be aware that just because oven cleaner is readily available, it isn’t less nasty, and you really don’t want it on your skin.
A variety of cleaners are based on citrus oils. You can purchase a spray-on cleanser similar to brake cleaner (TCE, TriChloroEthylene) but nontoxic. Some paint strippers are based on the same materials. These are indeed less toxic than the more chemically active products, but as you might expect, they do not work as quickly or completely. For light-weight jobs, these products are perfectly acceptable and kinder to your body and the environment.
Kerosene, Diesel, Biodiesel
Kerosene and Diesel fuel are oil-based, and they do a good job of dissolving oily crud. They are popular choices for benchtop and freestanding parts washers, and they’re safe to use inside your shop with reasonable fire precautions. Biodiesel is even better because the oils in it dissolve the grease better than the petrochemicals do. As with many other cleaners, keep heat, flame, and spark sources away from these flammable liquids.
I do not recommend using gasoline as a cleaning agent because it’s volatile and potentially dangerous. Although it effectively dissolves oily and greasy gunk, it’s also highly flammable and full of other chemicals, such as benzene and ethanol. Gasoline fumes are poisonous and easily ignite from any spark. If you do choose to use gas, never use it in an enclosed area.
Also remember that unsealed gasoline evaporates away, leaving behind a kind of varnish that you don’t want on your parts. Other products are far safer and work just as well.
Mineral spirits, generally known as paint thinner, do a great job of cleaning oily residues. There is a difference between the two though. Paint thinner is less pure, and often includes other petrochemical ingredients. Mineral spirits are far more expensive, but also far more pure. Both of them are related to turpentine, but that’s a vegetable oil–based product.
In general, mineral spirits are a much safer choice than gasoline for the same effect, and being purer it leaves less residue behind. Be aware that in an unsealed container such as a parts washer, mineral spirits evaporate in a few weeks, leaving an ineffective varnish-like liquid behind.
This solvent, like gasoline, is best left out of a large-scale cleaning process. Acetone is volatile, ignites easily, and produces a lot of vapor. Generally speaking, a small can of acetone can be best used for spot cleaning of resistant coatings such as polyester or epoxy-based paints, tape residues, and glues, including cyano-acrylate, commonly known as Krazy Glue or Super Glue.
Chemical Paint Stripper
Chemical paint strippers (as distinct from a citrus-oil version) are typically based on caustic soda (sodium hydroxide) or methylene chloride. Both of these are extremely nasty materials, so you should be sure to work in an open area and use your best gloves. However, they quickly strip old paint and varnish from parts.
It’s worthwhile to have a can of Jasco (which is based on methylene chloride) in your arsenal, especially for cleaning parts previously painted with two-part epoxy paints. Don’t leave any methylene chloride on steel or cast-iron parts for very long, as it attacks those materials.
Methylene chloride is safe to use on aluminum parts. Do not use caustic soda because it attacks the aluminum.
Trichloroethane (TCE) and per-chloroethane are used in brake cleaner sprays. These solvents are very effective at dissolving all kinds of grunge, but they’re expensive to buy in small cans. You don’t want to have prolonged exposure to these chemicals on your bare skin, or in an unventilated working area. It’s best to avoid them for major cleaning jobs, but good for removing oily residue in spots.
Chemical rust removers are nothing short of amazing in their ability to clean even deeply rusted parts. The several options include acid-based naval jelly, acid-based etching solution, and urea hydrochloride–based rust dissolver. Of these, the urea hydrochloride, such as Eastwood Rust Dissolver, is the gentlest to use in your home shop.
These products may be painted on as a gel or used in liquid form as a dip. These products work by reacting with the iron oxide (rust), and they eat into the metal a little bit. They continue working as long as they are in liquid form on the metal.
All of these products can be cleaned up and deactivated with plain water. You still need to wear neoprene or nitrile gloves and eye protection when you’re using them. Also use them in a well-ventilated area, because they produce fumes. Combining rust remover with scrubbing works very well.
As part of your preparation for powder coating, consider creating a phosphate layer on your metal. Phosphating is a process whereby an acid-etch treatment leaves behind a layer of phosphate mixed with the underlying metal. The acid and other ingredients in the phosphate solution create a chemical reaction with the surface of the metal that leaves a layer of iron phosphate crystals on the surface of the treated piece.
The phosphate protects the project from rust and enhances the bonding of the powder coat and the underlying part so it makes a great surface for powder coating. The result is a coating that is better looking and more durable, so your powder coat project looks better and lasts longer. The easiest way to create a phosphate layer is to use one of the many products on the market. These are all based on either tannic acid or 2-butoxyethanol and may also include phosphoric acid. You can use dedicated phosphating solutions. You can create a phosphate layer on bare steel by spraying any weak phosphoric acid solution over it.
It’s best to use these products just before powder coating. You want to coat immediately after the layer is formed. If possible, do not even touch the part before coating.
If you’re working with aluminum, a similar process is known as chromating. Either way, it’s best to consider this an immediate precoating treatment. Don’t leave phosphate sitting open to the air for long periods of time.
Commercial Chemical Dipping
Chemical dipping is primarily the use of a big tank of the same sodium hydroxide solution in some paint strippers. Although you can purchase a small amount and do your own dipping, this process is generally the province of professionals. If you’re thinking of coating a whole set of wheels or other large parts (or a large number of small parts), professional cleaning may be the easiest and most cost-effective option.
Dipping effectively gets rid of paint, body filler, and almost anything that isn’t steel. It does not dissolve lead, so if your part has some lead filler, you can use it. But the caustic soda completely dissolves aluminum, so don’t try it with anything that has aluminum content, such as pot-metal carburetors or other parts. Similarly, dipping agents, such as methylene chloride, are safe for aluminum but eat iron-based parts, so make sure you are using the correct chemical for the job at hand.
Project: Removing Multiple Paint Layers
A good time to consider commercial professional metal stripping is when you are removing factory paints or multiple layers. One cleaning project I undertook was a valve cover from an MG sports car that had several layers of paint on it. At least one layer was epoxy-based and did not respond to any chemical paint stripper. A flapper sanding wheel was not strong enough to cut the hard paint layer, so a wire wheel cup brush on an angle grinder was required to complete the job.
Once parts come out of a caustic soda bath, which has a chemical base, immersion in a muriatic acid bath neutralizes them. That’s the same acid added to the water in swimming pools. As a plus, the acid removes rust that a caustic bath can’t touch. Generally, the chemical stripping process includes steam cleaning and washing out of all chemicals used in the process. Be sure to ask about that when selecting a cleaner.
This MG valve cover had multiple coats of paint and lots of old glue and grunge on it. I wanted to see what it would take to really clean it.
Step one was paint stripper. I brushed on Eastwood gel stripper and it took the top layer of pink paint off in a thick, sticky paste.
I had to scrape off the pink goo. Whatever it was, it started setting up with the paint stripper gel and solidifying again! It could not be easily wiped away.
Once I got the pink goo off, I found a coat of light green paint. This was loosened with another coat of fresh gel paint stripper and scraped off.
Beneath the green was a coating of the original crimson color from the factory. This did not respond to the gel paint stripper at all, so I resorted to power tools to get down to the metal.
The sanding wheel did not get the job done on the flats, although it worked okay on the corners. I would have to get out the big guns.
A wire wheel cup brush spins at high speed and takes off anything that can possibly be removed. Be careful not to remove any more metal than necessary. Use this tool carefully on anything other than steel.
Here’s the valve cover with some clean metal finally showing! I still have a lot of work to do before this part is ready for powder coating.
Project: Powder Coating a Dirty, Unprepped Part
I, as well as many others, have stressed the importance of cleaning your parts, and this book is no exception to that chorus of unanimity. However, what most resources never show you is exactly why you need to clean every part thoroughly and what happens if you don’t. So, to settle the matter once and for all, I decided to powder coat an old, used part with no preparation at all.
I picked a coil spring from a rally car as a test subject. The spring was covered in the remains of old powder coating, rust, and a little dust.
Follow these steps:
Hang the old part with wire to facilitate an even coating.
Connect your powder coating gun and follow all regular coating procedures. Spray a good, thick coating of powder on the part.
Transfer the part to the oven and bake it, paying attention to proper curing time and temperature. When the part has cooled, take it out and inspect the results.
Although powder coating flows into some scratches and low places to hide a few scars, a finished powder coating is not nearly deep enough to cover the highs and lows of active rust and old, flaky powder coat. The results look terrible, and although powder coating is tough and wants to stick to the part, if the powder sticks to rust, dust, or other paints that are already flaking, it’s going to come off sooner rather than later.
After the bad results were photographed, I went back and did the job right. I spent about two hours using gel paint stripper to remove all of the powder coat, and then I spent some time with a bench grinder and wire wheel to take off the rust and remaining coatings. I recoated and cured again, and with proper preparation I achieved a smooth, even coat that is suitable for use on a show car.
When powder coating suspension springs, you need to closely monitor the curing temperature. You must not apply excessive heat during the curing process. If you apply too much heat, the springs lose their temper and the spring strength can be reduced. Curing temperature must remain below 350 degrees F.
This project may be stating the obvious, but it was worth it to see exactly what happens when cleaning is neglected. The point is this: Do not fool yourself into thinking that you can get the same results without cleaning your parts. Powder coating is easy to do successfully, but good results depend on good prep work.
Written by Jeffery Zurschmeide and Posted with Permission of CarTechBooks
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