Powder coating requires some very specific prep work if you want your finished product to be free of flaws and last a long time. The preparation steps below will eliminate 99% of any issues you may be having in your powder coating work. If you plan to start powder coating in the future, this step-by-step will give you an idea of what kind of prep work is required for successful coating.
Step 1: Disassembly
Disassembly is not always required, it really depends on what you are coating. If you are coating a piece of metal that is only made up of 1 part, then you can skip this step. If you plan on coating a part that has any rubber, plastic, gaskets, wiring, etc. involved, these pieces will need to be removed because they will likely melt in the baking process. Any bearings will also need to be removed, even if they are made completely of metal. Bearings are often packed with grease, that, when heated in the oven, will ooze out leaving the bearing useless or severely shorten its life. Magnets should also be removed as a lot of them will become demagnetized during the baking process. When disassembling anything before powder coating, just make sure that you will be able to properly assemble it when finished. Always do research on this before you even pick up a wrench. Anyone can unbolt a 2 brackets and coat them, but taking apart a transmission to powder coat the housings is a little more difficult. Always take pictures and notes of the disassembly if you aren't an expert on how it goes back together.
Even if you have an assembly that is entirely metal, but it is made up of multiple pieces, I often find that dissembling the part and coating each piece separately gives the best results. Some parts are more difficult to disassemble than others. While one part may be bolted together and can easily be taken apart, another part may use rivets, pins or other fasteners that need specialized tools to remove and replace. It is up to you how much you want to disassemble parts like these.
When powder coating car parts such as starters, alternators, master cylinders, etc., it is a good idea to purchase a rebuild kit for the part so you can take it apart, coat it, and put it all back together with brand new components. Just make sure you research if a rebuild kit is available and whether you should order it from the car manufacture or an auto parts store (usually the answer is always the car manufacture)
Step 2: Cleaning
After your part is disassembled, you will do a thorough cleaning of the part. You will need to remove all dirt, oil, grease, and grime from the part. Your goal with this cleaning should be that you could sit it on your best white shirt when done without it leaving a stain. Do not worry about rust or paint at this point, that will be removed later. Also some times cast metal items will be stained and no amount of cleaning will remove it, that will also be taken care of later. Definitely read this article which is a completely in-depth article on the cleaning step. A general degreaser can be used for this cleaning, just make sure it is safe for the part you are working with. For steel, I use and highly recommend Super Clean, formerly Castrol Super Clean. For aluminum, I use Simple Green Extreme, since it was specifically developed to be safe for aluminum. Along with the cleaners, I use an assortment of scrub brushes and an electric pressure washer on the larger parts. I chose an electric pressure washer simply because of the ease of use. This AR Blue Pressure Washer is a beast as far as electric units go and I am very happy with it. I have been using it for 4 years of almost daily use and had no issues. Previously, I had a Husky and a Karcher and this one puts those to shame. It is much more powerful and it has lasted 4 times longer.
Step 3: Stripping
After you given your part a thorough cleaning, you will then remove any previous coatings whether it be paint, powder coat, or anything else. If there are no previous coatings, you can skip this step. Depending on how much you powder coat, this step can be skipped and any previous coatings can be removed by sandblasting. If you are powder coating a lot, I highly recommend a stripper, especially if the previous coating you are trying to remove is powder coat. Some previous coatings can be a nightmare to sandblast off and the stripping step is a great time saver. I have written an entire article on stripping coatings and it can be found here.
Step 4: Outgassing
At this point, your part should be as clean as you can get it and down to bare metal. This is when you would do the outgassing procedure. Not all parts need to be outgassed so this step is "optional". Cast iron or cast aluminum are pourous and they can absorb oils during the manufacturing process or while they are in use. I always outgas all cast parts and any parts that have lived in a greasy environment. The outgassing procedure is the process of pre-baking the part in the oven at a higher temperature and longer time period than the actual powder coat curing. This pre-bake heats up the oils inside of the part and drives them out. You will often see a part smoking during this process, the smoke is a result of the oil burning up. If there are oils present inside of the part, and you do not outgas first, then these oils will come out during the powder coat curing which will leave little bubbles or bumps in the finish.
There is no set standard for outgassing, different coaters follow different guidelines. Some will leave the part in the oven until it stops smoking. Some will leave it in for a set time. My procedure is to put the part in the oven at 440 Degrees F and I adjust the time based on the size of the part. A small part like this starter would go in the oven for 30 minutes. For wheels, I would leave them in for an hour or longer depending on if they are still smoking. My reason for 440 degrees is simply because at around 500 Degrees, some aluminum alloys can start to anneal, which causes them to loose their heat-treatment, and therefore their strength. 440 Degrees seems to be low enough to stay far away from the annealing temperature, yet high enough that it will remove any oils that would come out during the powder coat curing. Again, there is no set procedure, this is just what I do and it works for me.
Step 5: Sandblasting or Phosphate Coating or Both
At this point, your part is ready for sandblasting. Sandblasting will clean your part to clean bare metal while providing a texture to the part which allows the powder coating to adhere to. When sandblasting, you will want to sandblast every single area of the part, completely, and thoroughly with clean media. There are some areas that should not be sandblasted, like brake caliper piston bores or cylinder bores. These areas need to be masked off prior to sandblasting. Do not use dirty or oily media to blast parts as you are prepping for powder coat. This oil will contaminate the part and you will see it in your finish. To see the full benefits of sandblasting and learn how to set yourself up for sandblasting, check out Media Blasting Part 1 and Media Blasting Part 2. If after reading those articles and you are still too stubborn to set yourself up with some sandblasting equipment, then check out the Alternatives to Sandblasting. The end result to sandblasting should be completely clean, white metal with no contaminants at all. From this point on, your part should not be handled with bare hands. Use clean gloves only.
Step 6: Post-Sandblast Cleaning
Sandblasting will remove almost all contaminates from your part, however your part will come out covered in sandblasting dust. Once you remove the part from the cabinet (WITH CLEAN GLOVES) you an remove a majority of the dust by spraying it off with the air compressor. Make sure you are spraying it with clean air though, you will need to have a moisture removal filter in the air line, other wise you can contaminate your clean part. If you want to learn how to get clean compressed air, check out this article. After a majority of the dust is removed, I will then scrub the part aggressively with a clean stiff bristled brush and blow it off with air again. Then I use denatured alcohol on a clean towel and gently clean/blot the part. Denatured alcohol is strongly recommended over acetone. It leaves absolutely no residues. I try to leave as little lint as possible during this step. I then blow off the part again and go over the part quickly with my trusty propane hand torch which burns off any remaining lint and also causes any traces of denatured alcohol to evaporate.
Step 7: Masking
Masking is dependent on the part. Some parts require some masking before powder coating and this is the time to do it. I do all of my masking with clean gloves on as it is the very last step before the part is powder coated and one sweaty finger print can show up in the finish. I do all of my masking on a clean workbench top with clean tools. If you would like to see some tips on masking, check out Masking Part 1 and Masking Part 2.
Step 8: Hanging
The last step is to hang your part, I use an assortment of hooks of different sizes as well as bare wire to hang parts. Bigger parts get a bigger stronger hook. The last thing you want to hear after doing all this work is your part falling down in the oven because the hook gave out. If you are powder coating your parts on a grounded rack, make sure the hook you are using is clean bare metal so it is able to conduct the ground through to the part.