You’ve performed the preparation work, the wiring, the endless parts cleaning, and the setup in your powder booth. Now you’re finally ready to apply powder! The truth of the matter: This is by far the easiest part of the process and the most fun. If you make a mistake or smudge the part, you can re-spray it so you’re just out a little bit of powder.
This Tech Tip is From the Full Book, POWDER COATING: A HOW-TO GUIDE FOR AUTOMOTIVE AND MOTORCYCLE PARTS. For a comprehensive guide on this entire subject you can visit this link:
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Powder coating is an art. You will learn to recognize when there’s a thick enough coating of powder on the part and how to deposit powder where you want it to go. This chapter is heavy on tips and advice, but there’s no substitute for learning to adjust your own gun and using it to achieve the best results.
I cover the process of positioning the parts and applying the powder, followed by handling instructions appropriate to the ovens and curing methods available to the home powder coater. Examples are provided for several different kinds of projects. These include some key examples of what not to do, what happens when you break the rules, and some of the unexpected things that can happen. Sometimes the unexpected thing that happens is unique and useful; other times, it’s just a mess.
The basic message is simple: Powder coating is easy, so have fun and be creative, and don’t be afraid to blow it all off the part and start over. Powder is relatively expensive, but not so expensive that you have to live with a mistake that you discover before baking your coating. Even after the coating is cured, it’s a simple matter of using some gel stripper and you can still start over again.
Masking and Plugging Parts
If you have some silicone plugs and the special high-temperature masking tape, you’re ready to mask and plug your part, if necessary. Basic items, such as steel bowls, wind chimes, wheels, and so on, often do not need masking or plugging. But you must protect any surface that you want to remain uncoated, and that includes the mating surface of an intake manifold and any threaded fitting. Powder coat goes on thick enough to make a threaded hole unusable without running a chase tap through the threads.
You can certainly use regular masking tape if you don’t have the special high-temperature tape, but here again you must apply the tape before you coat the item, and then remove it before you cure the powder.
You must remove ordinary masking tape before curing because the glue on the masking tape becomes rock-hard and adheres to the metal. You have to use a wire wheel to get it off, and if you did a precise job, you risk scuffing your new powder coat finish. You want to handle the part as little as possible between the coating step and the curing step. If you have to remove a bunch of complicated tape next to areas you want to coat, your chances of smudging the powder are nearly 100 percent.
On the other hand, the expensive high-temperature masking tape can go right into the oven, along with the silicone plugs. Then, when the powder is cured and the part has cooled off a bit, the tape comes right off and the silicone plugs pop right out and are ready to be used again.
One simple method to avoid smudging the powder is to place the part on a sacrificial cookie sheet, oven rack, or piece of sheet metal with the side to be left uncoated facing down. Then you can coat it and take it straight to the oven without touching it at all.
Another way to avoid smudging the powder is to apply tape to a part, then use a hobby knife or razor to cut out areas you want to cover with powder. If you use the high-temperature tape, you can coat and cure normally and then remove the tape.
For complex work with normal masking tape, try using your infra-red curing system or just some shop lights to melt the powder so that it flows a bit, and then remove the masking tape before finishing the cure in the oven.
Be sure to have the proper sup-plies before you start a project. Wax paper, baking parchment, and aluminum foil are excellent supplies to have in your powder coating workshop. When you need to cover large areas with something that is oven-tolerant, it’s better to use foil than a lot of tape.
Hanging Parts for Better Coating
If your project needs powder on all sides, you need to hang it from something to get all around it with the powder gun. You can buy expensive racks for this, but you can get great results with an ordinary folding ladder. You can buy premade S-hooks or make your own out of old wire coat hangers or wire fence clips. Prehang some S-hooks from the broiler element in your curing oven. It is far easier to hang a part from preplaced hooks in a preheated oven; you avoid burning your hands and smudging the powder.
Handling Coated Parts
Getting the part from your coating location to the curing oven is probably the most dangerous part of the process in terms of smudging the coating. You cannot touch any surface coated with powder, nor can you allow anything to brush up against the coated surface.
Some parts are easy to handle. If you coated a part that is on a baking sheet or in a foil pan, you can simply pick up the sheet or pan and slide it into the oven. If this is the case, you should preheat the oven to the curing temperature, slide the part in like a batch of cookies, set a timer to the cure time, close the oven door, and wait for the beep.
Other parts are easy to handle because you only coated one side; you can get a good grip on the part. For this reason, automotive wheels are usually easy, even though they are large.
Far more often, you face the intractable problem of having coated the entire surface of a part that now has to be moved into an oven where it barely fits. The challenge is to get it hooked to the broiler element and suspended, or supported from below, somehow, without smudging the powder.
First, test-fit your part in the oven before you coat it, so you have a plan for getting it in there. Then create hangers and supports as needed. Take your time and be careful getting the part into the oven. Do not preheat the oven; this way you can use your bare hands. The part cures just fine from a cold oven. It just takes a bit longer to come up to temperature.
Recovering from Mistakes
If you smudge the powder before it is cured, the repair is easy. If it’s a little smudge, you can just reshoot the affected area. Build up some powder and it flows together just fine.
If your part has several smudged or flawed areas, you simply disconnect your gun from the air hose, connect your blower, blow the powder off the part, and start over. This is usually the correct and best response, especially if you are coating a part you want to be perfect and visible when it’s in use. I’ve said it before, but it’s worth repeating: Powder is relatively expensive, but not so expensive that you have to live with a mistake that you discover before baking your coating. Bad results last a long time!
If you have already cured the part when you notice a flaw, it takes more time and effort than if the part is uncured. You need to use gel strip-per to take the powder coating off and start over. Although it is possible to add coats of powder, this does not generally cover up major errors.
Like so much with powder coating, cleanup is easy. For the most part, you just sweep up whatever ended up on the floor. If you use a drop cloth, you can just carry it out-side and shake it out. Powder is non-toxic and can be thrown away in the household trash.
It is important to clean your powder coating gun as soon as possible after every use, for several reasons. If you forget to blow your gun completely clean, you will have a blast of the old color in the first application of the new color the next time you use the gun. This will spoil the coat on the next part and you will have to blow it off and start over.
If you leave powder in the gun for any length of time, moisture in the air may cause the powder to clump and stick to the inside of the gun and its air passages. It takes only moments to blow your gun clean, and you should do it while your parts are in the curing oven.
Project: Powder Coat Application
Applying powder coat is extremely easy and effective in just a few steps. When you have a good, thick layer over all parts you want covered, you’re ready to cure the powder. Follow these steps for every project:
Lay down a drop cloth to catch your overspray. This is easier than sweeping up later. If you are working in your garage, cover your cars, workbench, and anything you don’t want covered in powder!
Make sure the part is completely dry and clean. Set it on a box, table, or rack, or hang it from some-thing so that you can work on it conveniently and without smudging it.
If you plan to use a baking rack or a cookie sheet in the oven, put it in place now. It’s okay if some powder gets on those items. They will end up with hundreds of layers of powder on them over time.
Be sure that you are setting up within a few feet of your work-space. Wires and an air hose tether your powder gun to the transformer and the compressor. You need to get all the way around your project part without dragging the wires over it.
Connect the ground wire to the part. If you start coating and you notice that the powder is not sticking, chances are good that you forgot to connect the ground clip!
Prepare your powder coating gun: Fill a cup about 1/3 to 1/2 full of your chosen color of powder and install the cup on the gun.
Connect the pressure regulator to the gun and the air compressor line. You can get short lengths of air hose that make this part easy. Set the regulator to a pressure that works well. Most guns specify 5 to 10 psi, but I found that 15 to 18 psi works better with my regulator.
Put on your respirator, goggles, and rubber gloves (if you choose to wear gloves). Any powder on your skin washes right off. But powder in your eyes itches for about a day, and powder in your nose and mouth has you coughing and stuffy for at least several hours.
Make sure you have the trigger in one hand and the gun in the other. You can also use your trigger hand to hold the air hose and wires out of the way.
Squeeze the trigger to activate the electrical current. Then pull the trigger on the powder gun. You should hear air fl owing through the gun and powder should start coming out immediately. If you don’t get a good puff of powder, shake the gun to get the powder moving in the cup. You can adjust the air pressure if it’s not working.
Working about 6 to 12 inches from the part, move the gun back and forth while dusting the surface with powder. Go all around the part and spray a good, even layer. When you think it’s too thick, it’s just barely enough. Be generous with the powder layer.
Curing the Powder Coating
Once the part is in the oven, the only thing you have to do is allocate enough time to let the part come up to temperature and cure the powder. The temperature and cure time vary based on the powder’s composition; it should be listed on the powder’s packaging. This is one place where using well-known powders, such as those from Eastwood, offers a big advantage. Some powders you buy on the Internet do not come with specific instructions about time and temperatures. They come in a reseal-able bag and you have to guess.
As a rule of thumb, you can expect solid colors to cure in 15 minutes at 350 degrees F. Clear or translucent colors should be cured at 375 degrees for a minimum of 25 minutes. How-ever, some powders have cure temperatures up to about 400 degrees, so make every effort to get instructions with every powder order. Purchase an oven thermometer at the grocery store to double-check the accuracy of the oven’s internal thermometer.
Once you have the part in the oven, resist the urge to check on it until the allotted time is up. Other-wise, you just let heat out and potentially compromise the cure. When the timer goes off, you can open the oven and let the part cool in place (recommended for really heavy parts that hold a lot of heat) or you can remove it and let it cool outside the oven.
When the part is cool enough to touch with your bare hands, you can test the cure by digging your thumb-nail into an inconspicuous place. If you can make a mark with your nail, it’s not cured. This is more often the case with heavy, solid parts that may not be able to come up to temperature in the standard time. Although the heat works to cure the powder on the surface, the cold metal is keeping the powder close to the substrate from curing.
You can leave a part in the oven longer than specified if necessary. Powder coat is more sensitive to the temperature than to the length of cure time.
You can also take a little solvent (such as Eastwood PRE Painting Prep) on a shop rag and rub it on the coated surface. If you can rub off some of the powder, it’s not cured.
If you have projects too large for a conventional oven, you have two choices: send it to a commercial coating shop or invest in an infrared curing lamp. These cost several hundred dollars and come in a variety of sizes, but you can cure just about anything as long as you’re patient about it. You set the part up in front of the lamp and systematically direct the heat radiation onto each portion of the project for the specified time.
Shop Lights and Heat Gun
As an experiment, I decided to attempt to cure a small part using ordinary halogen shop lights. If you have ever used these, you know how hot they become. I coated a thick steel washer with Eastwood gloss black and placed it on a concrete floor, then arranged three halogen work lights closely around it and facing the part.
After 30 minutes the surface temperature of the washer as measured with the handheld infrared thermometer was about 250 degrees F, and that was as hot as it got. Yet the powder passed every test for cure: It is not subject to ordinary solvents, it is hard, and it does not come off the part.
A standard hardware store heat gun can also be used to cure powder coat under certain circumstances. These guns have a variety of set-tings and can create temperatures between 250 and 1,000 degrees F or more. The risk is getting your powder coat too hot.
The challenge with heat guns is that they have blowers to transmit the heat from the gun to the part, and so the powder may be blown off your part before it is hot enough to flow out. But if you made the powder flow with your halogen lights, you can complete the cure with a heat gun. Just move the gun around a little to evenly heat the whole surface; be sure to work in sections on larger pieces.
Preheating to Help Cure
One technique that yields good results is to preheat your part before spraying the powder. If the part is heavy enough to hold heat, preheat it to about 350 degrees F in your curing oven, and then take it out and hang it or set it down for coating. Have your gun ready and apply the powder coat while the part is hot. The powder begins to melt and flow out as soon as it hits the hot metal, which increases the amount of powder that sticks.
Give the part the same thick coat you would give a cold part, and then place it back in the oven at its cure temperature for the specified cure time. You get a great, thick coating with a lovely finish.
Curing Multiple Coats
For some projects, you may find you need more than one coat. Some powders, such as metallics, benefit from a clear coat over the color coat. Some projects are complex and require two coatings to get everything covered and cured. Or you may have a desire to do a two-tone project.
In most cases, a multiple cure is not a challenge. Just remember to cure the powder with the higher curing temperature first, and don’t heat the part more than a few times.
Clear coats are the most common multi-coat scenarios, and they are straightforward. Hang the part or use whatever method you chose for the color coat, and cure the clear coat.
Metallic coatings are nothing short of amazing, but they need to be carefully applied and cured for best results. Preparation of the part surface is absolutely key. To achieve a chrome-type or gold-type finish, the part must be absolutely smooth before you apply the coat. Metallic finishes use real metal powders, and therefore are not as impervious to weather as other powders. Give your metallics a clear coat if you plan to use them outdoors.
I performed two projects with Eastwood Chrome powder and got excellent results. The first was a gar-den bell made from the bottom of an old steel welding bottle. This part was extremely heavy and a challenge to move, but the feature that kept the chrome from being as reflective as possible was the comparatively rough surface of the bottle. However, the surface is still nicely reflective and benefitted from a later recoat with a clear gloss to bring out the chrome color even further.
The second project was a small aluminum casting of a sailboat, which became the wind catcher for the bell. This project was also a solder test to remove an unwanted hole. The smoother finish of this piece yielded a much more reflective surface with the chrome powder.
Without a doubt, one powder that should be in your arsenal is East-wood’s black wrinkle-finish Wrinkle coat. It is often used on automotive parts such as intake manifolds. This high-quality powder gives effortlessly great results. There’s no trick; you just apply it and cure it according to the instructions on the cup and you have a perfect wrinkle finish.
The perfect complement to black is a red wrinkle coat, and I tested a different brand purchased on eBay. I tested the two wrinkle colors on two different parts. A pair of intake manifolds for a vintage VW engine received the black coating, while the intake plenum and throttle body to a Ferrari received the red. The wrinkle effect in the red powder barely came through, but otherwise created a good coating with a true red color. Unfortunately, Eastwood does not offer a red wrinkle coat at this time.
Miscellaneous Coating Challenges
Some projects present different challenges, including complex shapes, certain sheet-metal items, and alloy wheels. When powder coating these items, you need to take the correct steps so that powder is properly distributed on all surfaces, and as a result, the final finish covers the entire object.
From time to time, color change occurs during the curing process, usually with off-brand powders. This is sometimes associated with curing at a higher temperature than the powder needs. If you have ensured that this is not the case, it may simply be an attribute of the powder and there’s nothing to do about it but note the phenomenon and take it into account.
This is a good reason to always test your powders on scrap parts before coating a part that you really want to be perfect.
Complex shapes pose a particular challenge to the powder coater. Not only do you have to get into all the nooks and crannies with the powder, but the nature of the electrical charge that attracts the powder particles also behaves differently in valleys and holes. The powder is attracted to the ridges of the part, and you have to work carefully to get powder into the crevices. This effect is known as a Faraday Cage, and the practical effect is that your powder does not deposit evenly.
One technique that helps avoid this is to remove the diffuser disc from the front of your gun, and shoot powder directly into the holes. Alternately, hold the gun sideways with the diffuser disc in place, encouraging the sideways flow of the powder into the holes. In extreme cases, you can even use the gun without the electrical flow and rely on gravity to put some powder onto every surface.
A neighbor showed me a couple of sheet-metal lawn sculptures that had been cut out on a computer-controlled plasma cutter. Over time they had rusted, and he thought they would look better with a bright powder coating.
The first step was to get rid of the rust, which was accomplished easily with Eastwood’s Rust Dissolver, aided by a wire wheel on my bench grinder. Then it was determined that one should be red wrinkle and the other should be smooth orange.
Because they needed every point covered on both sides, I hung them from a ladder using safety wire. I wanted to avoid the tiny gaps in coating that result when you attach the grounding clip to the part, so I attached the grounding clip to the steel safety wire. This provides enough of a path for the electricity so that the powder sticks to the metal as usual. A little more testing found that the clip could be placed on any metal piece with an unbroken touching connection with the part, and the electrostatic effect worked.
Refinishing automobile wheels is a popular use of powder coating. Luckily, all but the largest wheels fit readily into a standard kitchen oven, so you can do this work yourself and get great custom results that hold up well to the wear and tear of driving.
Working on steel wheels is straightforward: Simply clean them of all rust and paint. Paint is easily removed with media blasting or chemical stripper. You can decide if you want to coat only the outward-facing surface or the back as well.
Wheels with complicated pat-terns may pose a challenge, especially if you want to limit your coating to particular areas. It’s best to plan on coating the entire outward-facing side.
Alloy wheels may be different from steel, however. Alloys are usually clear-coated at the factory, and at a minimum that coating must be scuffed up to provide adhesion to the powder. It should not damage the wheels to treat them with gel strip-per to remove any prior coatings, but test the stripper on a small area first to be sure.
There are rumors that powder coating alloy wheels weakens them because of the curing temperatures. Some alloys of aluminum (notably 356 aluminum) have a low temperature tolerance, but reports of weakened wheels are anecdotal and many automakers include powder-coated wheels as standard equipment. If you are concerned about weakening your wheels, you should avoid powder coating.
Powder manufacturers generally recommend against mixing powders of different colors because powders do not truly “mix.” Instead, the particles of the different colors mingle, often randomly and imperfectly, and create an effect of interspersed points of color between the two (or more) colors.
But this has an interesting effect on the human eye, which averages the colors it sees if the points are small enough. In fine art, the technique is known as pointillism and it creates a deep color.
I mixed up a batch of “purple” by combining Eastwood gloss red and Ford blue powders. This created an attractive mix that is never the same twice. When viewed from different angles, the same area of a part appears to change colors from more red to more blue. The effect is mesmerizing! (I used the same mixed powder on a frying pan for the preheating test to good effect as well.)
As noted many times, if you don’t like the results of your powder coating, you can easily strip the part and start over. So why not experiment with color mixing?
I experimented with a few two-tone projects because they are the most challenging. The most notable was a small galvanized tub suitable for tabletop use. Galvanization involves electrodepositing a layer of zinc onto sheet metal to help prevent rust.
To achieve a two-tone finish, I first laid a layer of deep green on the outside of the tub and cured that finish. Then I coated the inside white. To my surprise, an impurity (probably glue or sealant) appeared in the white coat on the seams of the tub. That impurity bubbled up and burned in the curing process, spoiling the finish at the seams. The interior really needs to be stripped and recoated. However, the powder has no issues adhering to the clean zinc coating.
The principle of two-tone coating is sound. Most powders on the market are thermosetting and do not melt twice. So if you can mask the part effectively you can cure in two stages and achieve a unique and artistic effect without much trouble.
Written by Jeffery Zurschmeide and Posted with Permission of CarTechBooks