10 Tips for the Perfect Powder Coated Finish
1. Communicate with your customer
The business of powder coating requires excellent communication with your customers. When you agree to powder coat something, make sure you get all the details correct. Providing an accurate quote is important. Also make sure you know the correct color and gloss that’s required. Find out if there is a particular mil thickness or salt-spray rating that the finish must meet. Is the finish for exterior or interior use? Is there a 3, 5, or 10-year warranty? Does the customer need a heat resistance rating? Understanding the end-use of the part is critical when you are doing a job for a customer. This can save you a lot of headaches at delivery and make for a completely satisfied customer that will spread the word about the fantastic coating service you supplied.
2. Clean your metal
Make sure all the oil, rust, laser scale, mil scale, weld splatter, and weld flux are cleaned off the part. Professional quality powder coated finishes require thorough cleaning. If any defects are under the finish, it will eventually flake off and expose raw metal to the elements. Poor cleaning and inadequate parts preparation is the number one cause of reworks in the powder coating industry.
3. Prepare your metal
For the most durable finish, make sure your parts are etched or blasted in such a way that the powder coating can stick to the metal. Coatings have a hard time adhering to smooth metal like extruded aluminum or machined steel, so make sure your coating has something to grab onto by blasting or using a chemical pretreatment process. If premium exterior durability is desired, this is the step where you can add an extra rust preventative treatment.
4. Ground your parts
Use clean hooks and racks as much as possible. Bury an 8-foot electrical grounding rod in or near the booth and connect a 14 gauge or heavier cable with a sturdy alligator clamp to attach to your racks. In a pinch you can use the grounding wire from the gun unit, but your powder will stay on the part better with a stronger ground. Set aside a little time each week to grind or blast the tops of your racks to get good metal to metal contact with your hooks. Also grind or blast a spot on your racks for the clamp to attach. You may want to add a bolt or section of rod to your rack that’s easy for the clamp to grab. Hooks can be used 3-4 times before they need to be cleaned or replaced. Hooks using diamond or square stock typically give better contact and leave smaller hook marks.
5. Use only clean, dry air at a consistent pressure
Air pressure supplied to your powder gun should be around 80 p.s.i. and the pressure needs to stay the same throughout the day. Install a secondary regulator and an oil/water filter before the gun. Be aware of other machines that use air in the shop because spraying powder at varying air pressures will give you unpredictable results. Blast pots are notorious for using large volumes of air, so check your gauge if you are blasting and powder-coating at the same time using the same compressor. Water in the air lines is bad for the powder application unit (powder gun) and causes problems with the powder. For best results, especially in warm, humid climates, add a refrigerated line dryer.
6. Use consistent gun settings
Make sure you use the correct setting for the part and the powder being sprayed. Some heavier powders need more air while lighter powders need less. More kVs will help build powder on flat surfaces but will make it difficult to spray corners. Powder suppliers have recommended starter settings that you can tweak for your process. If you have a professional quality gun (which you should) from Wagner, Gema, or Nordson, your gun will have standard presets for large panels, recoating, and Faraday cage areas, as well as presets that can be used for custom settings.
7. Spray your powder consistently
Typically you want to use a cross coat pattern and will need to spray about 2-5 mils DFT. This often requires 3 passes but can be sometimes be done in 2 or require 4 or more depending on your powder setting and the speed of the sprayer. You usually need to give extra attention when powder coating a part to make sure you reach and cover the back of the part area first, then spray the fronts or Class A surfaces last.
8. Check your work before curing
Use an LED flashlight and inspect your powder coated parts before baking. If you catch a defect at this stage, it is much faster, easier, and cheaper to fix it than if you find it after the powder is cured. Look for light areas of powder coverage, especially in visible corners. Make sure you have a uniform coverage on large flat surfaces. If your parts have channels or features that can collect airborne powder, make sure you don’t too much powder build-up in those areas. Check all surfaces to make sure the applied powder looks like worn felt. If you see heavy starburst patterns or excessive edge build-up, blow off the excess and lightly feather in the coating with your powder gun to achieve a more even finish.
9. Use the correct cure temp and time
Check the product data sheet from the powder supplier and find their curing schedule. Don’t forget to add in the time it takes for your parts to reach curing temperature. If you add the heat-up time to the curing time in the schedule, that should equal the total time the parts need to be in the oven. For example, if a 1/4” thick plate steel part takes 20 minutes to get to 400°F with the oven set at 420°F, and you see it takes 10 minutes to cure the finish according to the data sheet, you’ll need to bake the parts for 30 minutes at 420°F to meet the required curing time and temperature. As a general rule, it’s better to leave the parts in the oven a couple minutes too long than to risk taking them out too soon. If you have any doubt that the finish is cured, perform a solvent rub test on a sample part. If you don’t get to metal after 20 double rubs, then you are typically good to go.
10. Quality control your finished products
Solvent rub tests are great QC checks to make sure your parts are cured. You’ll also need a DFT (dry film thickness) gauge in order to accurately measure your powder thickness. You’ll typically want a finish that is 2 mils or thicker. If you find you are getting 8-10 mils, that’s probably too much unless the powder supplier recommends it or your customer specifies it. Do a cross-hatch adhesion test on a sample piece whenever you are testing new processes or trying new powders. You should also keep a record of defects such as trash, blisters, fibers, gloss variations, grind marks, fisheyes, pinholes, etc. so you really know what kind of problems you are having and aren’t just guessing or working from memory. Talk with your powder supplier or a coating expert and develop a plan to eliminate your most common defects one by one, rather than trying to fix everything at once. This will help you identify the exact causes of the issues you are having so you will know how to avoid them in the future.
Need Help? We’re Here for You!
In addition to manufacturing, installing, and supporting our own powder coating equipment, Reliant also provides a wide range of services to help your existing finishing operation run smoothly–from on-site troubleshooting to training, equipment refurbishment, and more. Call us today at (877) 418-5550.