An article on this very subject appeared in the June issue of
Austin-Healey Magazine. I'll paste it in here, below. By the way, I highly recommend
Austin-Healey Magazine for anyone with an interest in Austin-Healeys. At just $35/year, it costs about half as much as an hour of a professional mechanic's time, or about as much as two fill-ups for your Healey. You can join
HERE. /ubbthreads/images/graemlins/grin.gif
SANDBLASTING
Matt Parrish
Fort Smith, Arkansas
This article discusses the subject of sandblasting. Of course, other media besides sand are used, but the terms "sandblasting" and "mediablasting" are used interchangeably here. I will attempt to put this information into the context of automotive restoration, but first a word about safety...
The first key to proper sandblasting is safe sandblasting. Aside from the obvious dangers of shooting an abrasive out of a gun at anywhere from 35 to 90 PSI, there are certain other dangers that must be considered. So anyone undertaking the task should read and understand all safety precautions with their equipment and each abrasive medium they are using.
The decisions about which blasting medium and which equipment to use will be largely determined by your needs; each medium has its benefits and its drawbacks.
Equipment Types
To begin, the equipment you choose can be largely determined by your project. For instance, there are really only two main types of sandblasters: the siphon type and the pressure type. Both refer to how the blasting medium is picked up into the stream. The siphon type has an open hopper that contains the medium, and this is sucked in as the air from the compressor creates a vacuum exiting the nozzle, pulling the medium into the compressed air stream, so that the mixture can jet out of the gun and toward the target. The pressure type uses a closed and pressurized container, and the blasting medium and air mix a little below the hopper and jet down the hose together. Each has its advantages and disadvantages.
The pressure blaster is generally the more powerful of the two, however it is more expensive and finicky to use, and more prone to clogging and constantly needing adjustment. Also, once the system is going, it keeps on going. There is no easy on/off. The siphon may be less powerful, but its lack of a pressurized media hopper and simpler systems allow for a lower cost. And the system is still powerful enough for your typical restoration needs. It also offers a greater level of control, as there is a button on the gun that cuts off and releases the airflow, allowing the spray to be turned on and off more easily. This makes the siphon type the favorite of most home users, because really the only calls for the greater power of the pressure type blaster are stone carvers, glass artists, and others actually wanting to form the surface with the stream.
Nozzles
After you have chosen the type of your blaster, you need to consider your nozzle. Your choice will be delineated by the media and its properties, which in turn are determined by your needs. Different nozzles allow different air flows rated at cubic feet per minute for a given pressure. Media are rated for various pressures, based on the speed at which they are safe. The greater the pressure, the greater the volume through a given nozzle size, and the faster the media must move. The faster it moves, the more abrasion it can produce. Which brings us to another point in nozzle selection: durability. The harder the stuff the nozzle is made from, the longer it will last. This also means that in most cases that the price increases as well. The materials range from soft brass through the ceramics to the hardened metals. Take a good look at how much you will be using your blaster. Someone not doing a lot of blasting might forego the expense of a hardened metal nozzle and choose a cheaper ceramic or even brass if you have relatively little blasting to do.
Containment
Whichever type sandblaster and nozzle you determine are best for your needs, remember this: sandblasting makes a mess. The size of the mess is determined by the size of your containment equipment. The most basic of these is the cabinet. It isn’t hard to find or make a sandblasting cabinet of sufficient size for most of the individual parts of your car. This allows you to blast the target and keep the mess contained to a relatively small space. That also allows a greater level of safety for you. Any blasting media can be irritating to the lungs and eyes; keeping it contained will greatly reduce this risk to your safety. A good cabinet will be completely sealed when the door is closed, it will include a window allowing you to see inside to what you are doing, and it will have openings for the air hose and gauntlets that protect your hands. It will also be shaped in such a way that the media, removed paint and rust can fall away from the object into a collection bin. This will allow you to sift and recycle the media through the system.
However, there are parts that just won’t fit into your cabinet. They might be too large or too heavy, and these require a little special handling. The first consideration is still your safety, and a full suit is a good idea. These are typically made of Tyvek or some similar impermeable material, and these will keep the grit away from your skin. Top that off with a hood and goggles to protect your head and eyes. And don’t forget the gloves for your hands; "gauntlets" that come up past the wrist opening of the suit are your best bet. Remember, anywhere the grit can get, it will get. Lastly, but certainly not least, a NIOSH breathing apparatus is very important. While some media reduce the levels of dust significantly, none eliminate it entirely, and you want to avoid inhaling it.
Personal choice determines the rest of your containment. How much do you want to clean up? Personally, I would use plastic sheeting and duct tape to close off an area just as I would for a makeshift paint booth. It saves me trouble, and just as I don’t want to be at risk, I wouldn’t like to put anyone else in harm’s way by being irresponsible for the dust. There may also be local regulations to consider.
Surface Preparation
Now we can turn our attention to surface preparation. There may be areas of the target that you don’t want to blast, or where you don’t want the grit to go, particularly with engine and suspension parts. For this you will need to mask off those areas just as you would for paint; however, the mask you need is a bit different. Called "resist," it is usually a thick plastic sheeting taped to the surface. This covers most any need you would have for the restoration. Monument carvers and glass etchers may at different times use metal cut in a repeated pattern, but in an automotive setting there is little call for such expensive measures.
There are other little items of equipment you can purchase as needed, such as different nozzles, sifters, etc., but we have covered the basics of getting through the project equipment-wise.
There are many types of media, all suited to specific projects based on abrasive qualities, reusability, hardness and safety considerations. A word of warning: don’t use the sand that gives sandblasting its name. Sand is high in crystalline silica, and that is very dangerous. The crystalline silica can get into the lungs causing irritation leading to fibroid scarring around the particles. This condition is called silicosis. If these scars keep growing or increase in number, then breathing can become difficult, possibly even leading to death. Even if it doesn’t kill you, it leaves you more open to tuberculosis and cancer. Sandblasting is particularly dangerous as the particles not only shatter off the bits of rust and paint, but they also shatter themselves in the process, putting a much finer dust of silica in the air and leading to more acute and accelerated silicosis. These dangers are real enough that many countries such as Great Britain have banned the use of silica sands as long ago as 1950. It is highly recommended that you use only abrasives with less than one percent free silica. Sand is cheap, and that might make it a tempting option, but any savings are easily outweighed by the cost of the equipment to safely deal with it, or worse by the health risks involved in not dealing with it safely.
Media Options
That said, it is now time to discuss the major types of media available on the market. I will start at the more abrasive and move to the less abrasive. These levels are important to your project as the more abrasive the medium, the more material it removes, including what you don’t want it to remove. There will be a certain level of "scoring" to the metal with most any system and blasting medium. This is normal, and even desired in some cases to help paint adhere. But it is possible to take this light scoring too far and make it difficult to get a smooth paint finish. Also, on sheet metal parts, the heat from the friction of the blast can cause warping of the panel, just like welding or brazing. This is not even close to a comprehensive list of potential problems. Rather, the goal is to give an impression and basis for comparison.
For any medium, there is a Material Safety Data Sheet, or "MSDS" for short. This is extremely valuable to you. It will not only warn you of specific hazards, but many of the products on the market come as brand names, such as "Green Diamond." From the name you can't tell what it is composed of, but the MSDS lists clearly what is mixed to make the product.
Silicon Carbide is one of the most aggressive media. There really aren’t many parts of an automobile that call for this level of blasting. Most often it is used in carving monuments, sculptures and glass. Like the pressure blasters it is very good at shaping the target material. This is good for the artist working in glass or granite, but not so good for the one working in sheet metal. However, it can be used with care on tougher parts such as wheels, manifolds and the like.
A good option to replace Silicon Carbide is Aluminum Oxide. While not as large a grit as Silicon Carbide, it can operate at the same pressures and is only a little less abrasive. This makes it good for cleaning stronger parts, such as manifolds and other cast parts. Aluminum Oxide is also very recyclable, able to be reused many times before it breaks down too far to abrade well. It does come with some safety issues, so again, be sure to read and follow all precautions listed in the MSDS. This includes protection for skin, eyes, and lungs, with the lungs being particularly important, as there are toxicity issues.
The best all-around blasting medium is Glass Bead. While not the best for any individual purpose, it will work well at them all. It also produces less dust than its counterparts, and is more gentle on the softer metals such as aluminum and brass. This also makes it good for cleaning pistons and engine blocks where removing surface material can be an issue. But special caution must be used to be sure that all the abrasive is removed prior to re-assembly to prevent scoring and premature wear. Glass beads are also much safer, requiring a lower grade of filtration, and they are not toxic like Aluminum Oxide.
Walnut Shell is a medium that is very good for internal engine components. It is able to cut paint, light rust and carbon well. What makes it especially suited to internals is its softness. You should also consider that it is not easy to be absolutely sure that you got every scrap of media out of every nook and cranny, particular on mechanical components, and this is where walnut shell comes into play because if any is missed in the cleanup, it is far less likely to cause damage. It will most likely break down before causing any problems. It also will not spark. This can become important if you find yourself working on parts of the fuel system.
The most gentle media are the poly abrasives. Made of soft man-made materials, polys are very common for working on fiberglass and the softer metals. Because it cleans with the least damage to the surface, it can be very forgiving. Gentle and reusable, they can also be very expensive compared to the other media.
One more thing: most media are available in a wide range of grit sizes. In general, the larger the grit, the stronger its cutting ability, and as you recycle a medium, its size will diminish as it breaks down. A good rule of thumb might be to choose as you would a sandpaper grit, starting with the more coarse grits and moving down to finer and finer grits.
Conclusion
Sandblasting can become confusing and daunting, but is well within the range of the home mechanic who is willing to take some care and learn. If you are nervous about your skills, get a book or some instruction, then slip down to the junkyard and pick up some practice targets for experimentation. This will allow you to build confidence without warping or putting unwanted holes in your dream car.
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