Why Sandblasting is the Best Method to Prepare for Powder Coating

Media blasting also known as sandblasting is a great preparation method for powder coating.

When powder coating, it is very important that the item you are coating is completely clean of oils, dirt, paint, rust, corrosion, and damaged plating (zinc, nickel, chrome, etc). All of these things can interfere with the powder coating process causing imperfections in the finish or causing it to fail completely. After cleaning your item of all oils and dirt, a media blaster does the job of removing rust and corrosion very easily and thoroughly when compared to other methods.

Powdercoatguide.com is a participant in the Amazon Services LLC Associates Program, an affiliate advertising program designed to provide a means for sites to earn advertising fees by advertising and linking to amazon.com.  

You can see how effective sandblasting is at cleaning metal here:

The main advantage that sandblasting offers besides the deep-cleaning of metal when preparing your items for powder coating is that 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 which increases the available surface area for paint and powder coat to stick to. 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 sanding with sandpaper can give your object a profile, it is much more tedious and sometimes difficult for complex items, 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.


  Imagine that this picture is a microscopic view of a powder coated surface and decide which surface would allow the coating to adhere better.

A sandblaster 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 sandblasting guns and nozzles that can be used outside of a cabinet, such as in a blast room, or just anywhere outdoors. However, these require that you wear some personal protection equipment to keep your skin, eyes, and lungs safe.


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 DIY sandblasters out there state that you need at least a 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 large 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 VS. 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. 

Sandblasting Cabinet

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.

With a sand blasting cabinet, the blasting process takes place inside a completely enclosed box. There is a window for you to see what your doing and long heavy duty gloves that are sealed to the front of the cabinet. The cabinet has a grate that you will place your items to be blasted on, and below that is an area for the blasting media to fall down into. Blasting cabinets are usually paired with siphon feed guns. The media falls through the grate to the bottom of the rack as your blasting, the media pick up tube is positioned down in the base of the media and the other end connects to your gun. This way you can blast endlessly and your media is constantly recycled. You will need good lighting to see what you are doing and also some type of dust collector. As you are blasting, the media will break down into finer and finer dust which constantly floats around making it very hard to see. The dust collector will suck this fine dust out of the cabinet which increases visibility. There are purpose built dust collectors and there are some DIY methods such as hooking up a shop vac to the cabinet.  The easiest method is to buy a quality sandblasting cabinet that comes with a dust collector and lighting. The Skat Blast cabinets sold by TP Tools are one of the best cabinets out there. The sandblasting cabinet pictured comes with everything you need to start blasting and it roomy enough to do wheels and other large parts.  There are cheaper sandblasting cabinets available if you do a little DIY work.  The cheaper cabinets usually need sealed up with caulk and some kind of DIY dust collector will need to be implemented.  Read how to setup some DIY dust collectors in "Media Blasting Part II"  If you have the room in your garage, I recommend a stand alone blast cabinet over the bench top blast 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.

The best media for sandblasting is not sand

There are many type of media to choose from when blasting. Having the ability to sandblast 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 much longer. There is also much less dust produced with aluminum oxide than with coal slag. This leaves a great profile in a steel or iron part to be powder coated. If you are doing any ceramic coating, aluminum oxide media is required to achieve the correct surface profile.  Keep in mind that because this is can be a more aggressive blasting media, that it can leave too much of a profile on softer metals such as aluminum. If the profile is too heavy, it may be visible through the powder coating finish, giving it a grainy appearance. Lowering the air pressure and keeping the sandblasting gun further away from the surface of the part will help with this.

Sand: Sand is the cheapest but I never recommend using  sand. 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