Another advantage of using a sandblaster to prep your items for powder coating is it gives the item a surface profile or "tooth". Think of sanding a piece of metal with sandpaper and you see the thousands of little scratches that it leaves. These scratches create high points and low points in the metal. This is called a profile. A profile allows the powder coat to seep into all of these high and low points which allow it adhere to the part much better than if the metal was completely smooth. While sandpaper does give your item profile, the sanding marks are in straight lines, a sandblaster creates a uniform textured finish all over the entire surface of your part, and in places that no sandpaper can reach. Without a surface profile, powder coating can just peel off the surface, this is called delamination.
A sandblaster is usually gun shaped and it uses high pressure air to propel sand or other media. When this media hits the metal, it can knock off rust, scale, dirt, paint, or other coatings giving you a clean bare metal surface. This sandblasting gun can be placed inside of a sealed cabinet to contain all your media and allow you to reuse it. This cabinet is called a blast cabinet. There are also free standing sandblasting guns that can be used anywhere, but with these, you must wear some protective gear.
To use a sandblaster, you will need an air compressor to supply you with this high pressure air, and a sandblaster consumes a lot of air so you will need a decent sized compressor. To see what kind of compressor you need or to see if your compressor is up to the task, take a look at "Choosing an Air Compressor" Many sandblasters out there state that you need at least an 2 stage 80 gallon air compressor that puts out at least 15cfm@90psi to sandblast. Air compressors are not cheap, even used, so while a huge compressor like that would be ideal, you CAN make due with a smaller compressor. The general rule is to buy the most compressor you can afford, but if you can only afford a 5 gallon pancake air compressor, keep saving! I personally use a Husky 60 gallon air compressor that puts out 11cfm @90psi which is enough to blast continuously, however the I do stop to give the compressor breaks.
Sandblasting also requires very dry air from the air compressor. Otherwise the media mixes with the moisture and causes frequent clogging. Another downside to this is, is that when powder coating, you want your part to be perfectly clean after sandblasting. If you are blasting with oily, moisture laden air, then this will likely cause defects in the finish. See How to Dry Compressed Air for more info on this.
Siphon Feed or Pressure Pot Sand Blaster?
As far as sandblasters go, there are 2 main types: "Siphon Feed" and "Pressure Pot". A siphon feed sandblaster generally looks like a gun, it can have a trigger or a foot control pedal. The gun has 2 connections. One is an air supply line from your air compressor and the other is a media pick-up hose which is a larger diameter hose that you can stick in a bucket of media. As the air passes through the sandblast gun, it uses a venturi effect to suck the media through the media hose and force it out of the nozzle of the gun. This style of sandblasting gun is cheap and gets the job done. Another great thing about these sandblasting guns, is you can stick the pick-up tube in baking soda and use it soda blast. It is not quite as efficient as a dedicated soda blaster, but I have used it to soda blast 100 pounds of nuts and bolts with no problems.
This Skat Blast Sandblasting Gun is the gun in my cabinet. This is the best sandblasting gun I have used and it is worth the price. The long handle makes it very comfortable to use since it requires very little effort to keep it depressed. It also feels more powerful than cheaper sandblasting guns and makes quicker work of sandblasting jobs. It comes with a steel nozzle, but after it gets worn out, I recommend using ceramic nozzles as they last much longer, or even better would be a carbide nozzle as it will last a very long time.
There are different versions of the gun and nozzles available for different air compressors:
Air Compressor Output Gun Nozzle
4-9 CFM S-35 Small Small Ceramic
10-15 CFM S-35 Medium Medium Ceramic / Medium Carbide
20-25 CFM C-35-S Large Large Ceramic
25+ CFM High-Volume Gun High-Volume Head
I also have a Campbell Hausfeld gun as a backup gun. It is cheaper but it seems to make the entire sandblasting process take longer. With all things being equal, the Skat Blast gun just finishes jobs quicker than the Campbell Hausfeld gun. The trigger is also much shorter on the Campbell Hausfeld gun so it requires more effort to depress it. After sandblasting for awhile, it is noticeably less comfortable than the Skat Blast gun. That being said, it is still a good gun for the price. It comes with its own media tube so you can use it inside of a blast cabinet or you an stick the tube in a bucket of media and use it anywhere. It also comes with a ceramic tip which lasts much longer than steel tips. However, if you plan on sandblasting on regular intervals, I highly recommend the the Skat Blast gun instead.
A pressure pot blaster generally looks like a propane tank with legs. It has a lid on top to pour media into and a smaller valve on the bottom where the media exits and goes to your sandblasting nozzle. The air supply from your compressor actually goes to the tank and to the valve at the bottom. This way the media is pressurized. This way the media is being forced out of that valve at the bottom and then the other air line is forcing that media + air to your sandblast nozzle.
A pressure pot blaster is more efficient than a siphon blaster. It uses less air and blasts about 4 times faster than a siphon-fed blaster. If you have a big job to do with a lot of surface area, a pressure pot is the way to go. However, they are more of an "outside" or in a "blasting room" tool. Once the tank runs out of media, you will have to either add new media, or sweep up the media on the ground and pour it back into the tank. If you recycle your media like this, make sure you filter the media somehow. I have a piece of widow screen that I folded over itself to make it filter more finely. A small rock can clog the machine.
Now that you understand the types of blasters, there are several environments where you can do your blasting. Blasting is a messy process. It creates lots of dust that will coat an entire garage, it throws media everywhere, and the media that hits tends to sting a little. Blasting can also be dangerous, a powerful blaster can take off skin. Media bouncing back at you can cause damage to your eyes, and the dust can damage your lungs, especially if using sand as your blasting media. So keeping that in mind, you can blast outdoors with some personal protection equipment, you can dedicate an entire room to blasting which will also require protective gear, or you can use a sand blasting cabinet.
Here is a bench top sandblasting cabinet. They are smaller, and don't have the funnel on the bottom that lends to easy media changes. However they can sit on a bench in the garage, taking up less room.
What Media to Use?
There are many type of medias to choose from when blasting. Having the ability to blast opens up a lot of options in the garage, not just for powder coating. For now, I will talk about the type of media that you want to use for powder coating prep.
Coal slag: This media is very cheap, it leaves a good profile, and it lasts a pretty long time. It is great at stripping paint and rust very fast. Examples of coal slag is the popular brand: Black Diamond, its sold at Tractor Supply Company. 50lbs is about $7.
Aluminum Oxide: This is a very aggressive media that easily removes rust, paint, and is more efficient at removing powder coat than the coal slag. It is pricier, but it lasts longer. This leaves a great profile in a steel or iron part to be powder coated. If you are doing any ceramic coating, you must use aluminum oxide to prep. Although when blasting aluminum, you need to be careful not to leave too much of a profile, as aluminum is much softer than steel. Hold the gun back further and lower the air pressure.
Sand: Sand is the cheapest. You can get a bag of play sand from Home Depot for next to nothing. However, sand is very dusty, making it almost impossible to see in the cabinet. Sand also produces silica dust when blasting which can severely damage your lungs even when using a cabinet. The only safe way to use sand is with a supplied air breathing system.
To continue reading about media blasting, click here: Media Blasting Part II