Sand Blasting Basics: - Surprisingly useful capabilities for home and shop

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Category: Shop

Surprisingly useful for home and shop">

by Ed Lance

Just about everyone is familiar with the term "sand blasting." Lots of folks, though, aren't aware of the many uses it provides and the multitude of blasting media, equipment and setups available. From small, enclosed cabinets to large, total-loss systems capable of literally stripping away metal and concrete, if you can imagine "blasting" anything, a system is available that can do it. Some forms of blasting even make use of liquids as a carrier for the abrasive grit and can be used to "cold cut" very hard metals such as titanium...several inches thick! Most of us however are familiar with the basic air and sand type blasters used to remove rust and old paint. Surprisingly, with just a change in air pressure and/or the blasting media, our basic blasting gear can be used in the home and shop to achieve some pretty interesting affects on our work pieces.

This brief article isn't designed to teach you the ins and outs of blasting, but rather to give an idea of what is available and some of the possibilities it can provide you beyond very basic rust and paint removal. We will discuss some of the basic equipment available and the various blasting media on the market and their uses.

Blasting equipment generally falls into two basic categories: "Recycling" and "total-loss," depending on whether the abrasive grit is retained or spent.

Recycling systems. Enclosed cabinets that keep the blasting media contained and allow its reuse until such time it requires replenishing. They are generally used for blasting small pieces (restricted by the physical dimensions of the cabinet), although very large recycling systems are available, they are very cost prohibitive for the home shop. Recycling systems also have the advantage in preserving expensive blasting media for continual reuse. Recycling systems however cost more than most open air systems.

Total-loss systems. An open air system usually consisting of a siphon or pressure feed gun that shoots the media onto the work piece and then scatters about on the ground. Open air systems on the other hand can be purchased for a pretty cheap price...as little as $12.00 for a basic, small siphon feed gun. The downside is total media loss after only one use. These systems are good for blasting large pieces and for use with cheap sand or silica based media (literally available right out of your own yard if need be).

We can further break down the systems into two more categories: siphon feed and pressure feed systems. Again, each system has its pros and cons:

Siphon feed systems. Open-air guns that are by far the cheapest of the lot. They are also the lowest performers on the list in terms of blasting power. They operate on the same principle as small siphon feed air brushes do for painting (the "Critter" gun being a fine example). Air is passed through the gun and over the siphon tube opening. The low pressure literally sucks the blasting media up the tube and into the air stream where it is ejected from the gun nozzle. While cheap and easy to operate, these guns tend to be prone to clogging, especially if the media is moist or you do not have a very dry air supply from your compressor. Expect to use high air pressures to achieve good results.

Pressure feed systems. Use a separate pressure tank to contain the blasting media. They are usually sold in sizes based on the amount of silica blasting media in pounds that the pressure tank can hold (such as a "40lb system, 100lb system, etc). These systems are more efficient than siphon feed systems and as a result, you achieve higher blasting power per pound of air pressure from the compressor. The downside is a system more expensive than a siphon feed and they are just as prone to clogging. If severely clogged, they require a lot more work to clear than a siphon feed system.

There are many, many types of media that can be used in a blasting system. The type of media is dictated by the material being blasted and the desired result of the blast. Some of the available media is listed below with their general uses. This is by no means an exhaustive list of the blasting media available, but only a sampling of the most common types in use.

Brown aluminum oxide. Widely used as a cutting media. It can produce an "anchor" pattern in preparation for recoating. It's excellent for removing heavy foreign matter, deburring, frosting glass and lettering stone. It is extremely fast cutting, can be reused many times and is classified in various sizes for a wide selection of finishes.

White aluminum oxide. This high purity aluminum oxide is utilized primarily in the production of grinding wheels and honing stones. White aluminum oxide is also used as a blasting media when iron contamination is a concern. Blasting applications include dental labs and aerospace parts. White aluminum oxide also finds use as a refractory material in the investment casting process.

Black silicon carbide. Black silicon carbide grains are produced in an internal resistance furnace from a mixture of high purity silica sand and carbide at temperatures as high as 4500°F. The hardness and sharp particle shape of this material make it an aggressive abrasive ideally suited for use in grinding wheels, honing stones, coated and loose abrasive applications.

Green silicon carbide. Green silicon carbide is a specialty grain used almost exclusively in the manufacturing of grinding wheels, mounted points and honing stones. The green color is a result of higher purity than black silicon carbide.

Glass beads. Available in a wide range of sizes (screens), glass beads are generally the most popular media used in most cabinets today. This all-purpose media is used for honing, polishing, peening, blending, finishing, removing light burrs, and cleaning most light foreign matter such as carbon and other surface residues from pistons and valves; and with no base-metal removal or dimensional changes to the part. Weld and solder flaws can also be detected via glass bead blasting.

Steel shot. A solid, round particle causing a peening action and producing a dimpled surface. Its heavy weight gives greater impact and hammering action for peening and cleaning heavy forgings and removing heat-treating scale.

Steel grit. Angular, and like thousands of tiny chisels and cutting tools, does a fast cleaning job. Produces an ideal surface for adherence of a new coating. Often shot and grit can be mixed to achieve unique finishes.

Corn cob. Known as soft-grit blasting, this natural cellulose grit is derived from corn cobs and is used for removing light foreign matter such as carbon, oil and dirt without affecting surfaces.

Walnut hulls. This soft aggregate is used in blasting processes for removal of foreign matter or coatings from surfaces without etching, scratching or marring the cleaned areas. Examples of applications include cleaning of delicate molds, armatures and electric motors prior to rewinding.

Plastics. This relatively new, dust-free media is a special formulation of plastic materials that has high tensile, compressive and flexural strength, combined with comparatively low hardness. Used for deflashing plastic parts and cleaning molds, dies, electronic connections and circuit boards. It can effectively deburr machined-iron castings and nonferrous screw machine parts.

Ceramic shot. Ceramic shot is designed for surface treatment application such as cleaning, satin finishing, deburring and tumbling. Ceramic shot is dustless and presents an exceptional working life. When used for stainless steel and nonferrous metal surfaces, no subsequent decontamination process is necessary. Excellent for stainless steel weld cleaning.

Baking soda. A soft single pass media used primarily for cleaning and depainting. Popular applications include machine, die, mold, and press cleaning. Maintenance uses include degreasing and stripping of almost any material without damage to the substrate.

Novaculite. A naturally occurring silica product intended primarily for wet blast applications.

Silica sand. An inexpensive non-recyclable media used for general cleaning, rust removal and surface preparation. Silica sand is most popular for use on ferrous materials. Negative aspects of using sand include large volumes of waste material, excessive dust and health concerns. Not recommended for use in cabinets.

You might be wondering by now what a basic blasting system can do for you besides clean metal and remove old paint. The answer to that is really up to your imagination. Very tiny grit media is great for frosting glass. Use of some sort of resist (vinyl tape, for example) to mask the glass makes for great looking frosted patterns. For a really neat effect, blast the backside of a mirror with a pattern and then backlight it! Try some blasting on some of your wood projects. You'll be amazed at the finishes and textures you can achieve. Want some pizzazz on your rather plain brass hinges? File the leaf edges to a fancy pattern, then use steel shot to blast them for a hand peened effect. The possibilities are endless!

OK, so how much will a blasting setup cost me? Well, that all depends. As mentioned, large pieces (usually things that are more than a couple feet in length, width or height) are done outside using a total-loss gun of some type. You can purchase one of these like I did at Wal-Mart for as little as $12.00. If you have small parts you want to do, and want to use some of the more expensive media, a small bench-top blasting cabinet can be had for as little as $79.99 at Harbor Freight.

Let's also talk quickly about air supply and air requirements. As mentioned, siphon feed systems are at the bottom of the food chain in terms of blasting power. A lot is decided by the blasting media you use, but you will still need sufficient air pressure and volume to drive the system. I have a 30 gallon, two-cylinder Campbell Hausfeld compressor and it does OK with my cheap siphon feed gun, as long as I don't expect to run the blasting gun continuously. Large pieces can be a bit time consuming as I blast, stop, blast, stop, etc. The small bench top cabinets run just fine on this size compressor. Anything larger is going to require a bigger compressor for efficient operation.

Clean, moisture free air is one of the most important aspects of blasting. Water in your air supply is a sure recipe for frustration as you begin clogging your gun. There's no fun in cleaning clogged lines and guns. Pay a few dollars for a good water filter and you'll be fine. Also ensure your blast media is clean and dry as well.

Lastly, and most importantly we must talk about safety. ALWAYS wear a respirator when blasting, especially when using any type of glass or silica based media. You think wood dust is bad for you, try breathing in fine glass or silica dust...it's flat ugly! Always wear good protective clothing, gloves and goggles, or a full face shield. High speed particles bouncing back can eat the skin right off your knuckles or other parts if you don't! (Don't ask me how I know this!)

Tastefully done blasting can add a whole new dimension to your woodworking and craft projects. I highly encourage you to give it a try.