When powder coating, media blasting (sand blasting) is one of the best preparation methods possible. It cleans, de-rusts, and it leaves behind a texture that makes powder coating stick like nothing else. Read this Media Blasting article to realize the full benefits. But for the stubborn people out there (I was one of them) who don't own a big air compressor or sandblaster and don't want to invest in them, I will go over some alternatives to sandblasting. Note: The alternatives are slower and do not achieve the same durability in the powder coating finish as media blasting.
There are some parts that I will still use this method on. Valve covers and oil pans with internal baffles like to trap blast media. Unless you remove the baffles to thoroughly clean out all of the media afterwards, media will get trapped in the part. Later when these parts are installed in the engine, the media can get washed out by the oil and circulated around the engine. Blast media will quickly score bearings and journals inside the engine which can result in an engine rebuild. Not worth the risk to me to media blast these types of parts.
Assuming the part you are planning to powder coat is not brand new, it probably has old paint, rust, or corrosion on it. To prepare a part for powder coating without a sandblaster, the following are different methods that I used.
The first step I used was a bench grinder with a wire wheel.
Note: if your part is stainless steel, only use stainless steel wire wheels.
This is a Dewault 8 inch bench grinder: (It also also works great for polishing aluminum)
The wire wheels are good for removing paint and rust. They get old dirty metal down to clean bare metal. They take much longer than a sandblaster, and I was okay with that at first but it becomes tedious. Also the 8" wire wheel will not reach all the crevices.
After finishing all the larger open areas with the bench grinder, I then switched to smaller wire wheels mounted in a drill. They work great at getting into almost all of the small crevices.
This is around the time when I setup my make-shift peg board to keep myself organized. Now that I use a sandblast cabinet, the board has been decommissioned.
After I got into all of the areas I possibly could with the various sized wire wheels, I used a Dremel with a flex shaft and a very tiny wire wheel . The little wire wheels wear down very fast so I learned to stop buying the Dremel name brand ones that were $6.00 each and found this assorted pack which comes with every size you need to get to any crevice and they are much cheaper. Just keep in mind that they fling sharp bristles as you work. Protect your eyes and skin.
Another simple way to keep organized was this make-shift Dremel attachment holder. I hated the Dremel case so I just drilled 1/8 holes into a block of wood, and now my Dremel attachments are always handy. The container is held onto the wood with double sided tape. Little things like this keep me sane in the garage.
So now that you have spent a couple hours wire wheeling your way to clean bare metal, you should have a part that went from this:
The next step is bound to be your favorite: Sanding.
While sanding, your aim is to remove the shine. I always used two consecutive steps, first 3M 220 Grit and then finish it off with 3M 400 Grit sandpaper. Remember, the goal is to add texture, not to remove a bunch of metal and change the dimensions of the part.
After sanding, you can then re-clean your part and go on to powder coat it. However, keep in mind that when you rinse any cleaner off of the bare metal part, the part will want to rust very quickly. It is best to do the final rinse in cold water and dry it as fast and as thoroughly as possible. I recommend wearing disposable gloves while drying and handling the part after it is clean. Touching the part can cause little rusty fingerprints to show up due to the salts on your skin. After the part is dried, do a quick bake in the oven to remove any remaining moisture.
I still found the powder coating was less durable than desired so I broke down and bought a sandblasting cabinet and an air compressor. Not only is it way faster, the powder coat was much more chip resistant. I still wanted to cover these methods for those that are not ready to invest in blasting equipment yet. If you are planning on powder coating parts for paying customers, absolutely do not use this method, sandblasting is the only way to go, read this Media Blasting article to realize the full benefits.