Sandblasting is one of the easiest and quickest ways to remove rust, old paint, dirt and grease from metal and other hard surfaces. With the proper abrasive media, this process (sometimes referred to as bead or pressure blasting) can strip years of corrosion from plate steel, yet it can delicately etch glass or carve wood.
Sandblasting is a perfect method for restoring intricate metalwork such as wrought iron fencing or for removing layers of paint from the numerous nooks and crannies in old cast iron radiators. Best of all, the basic techniques and tools are easy to master and fun to use.
Types of sandblasters
A sandblaster works on the same principle as a paint sprayer. Compressed air at high pressure blows fine sand or other abrasive material through a hardened-steel, carbide or ceramic spray nozzle.
There are two types of sandblasters: siphon- (or suction-) feed and pressure-feed. Both require a steady supply of compressed air, so you’ll need a compressor capable of delivering 70 to 125 psi at 6 cfm or greater.
Siphon-feed blasters (the type we used for this project) are designed primarily for hobbyists and home users. They cost as little as $60, but they’re less effective than pressure-feed units because they use part of their energy to suck abrasive material up to the gun outlet before the blasting begins.
On the other hand, pressure-feed blasters use a pressurized tank to force abrasive media up through a flexible pipe at high velocity. Designed for professional users, they range in price from about $500 to more than $3,500 for large industrial units.
Though additional sandblasting equipment (such as an air dryer, a blast cabinet and a reclamation unit) can make the process easier and less messy, it isn’t necessary for occasional sandblasting. If you plan to tackle more than occasional projects, you may want to consult a local sandblasting equipment dealer (check the Yellow Pages) for advice on these additional components.
The effectiveness of a siphon- or pressure-feed blaster at removing rust, paint or scale is greatly influenced by the type of abrasive material you use. A medium that’s too light will take a long time to work, whereas one that’s too heavy can damage the workpiece (see Chart and Photo 2, above right). Common types of abrasive blasting media include:
• Silica sand — This medium is typically used outdoors for rust and paint removal. Because it contains free silica, which is a respiratory irritant and a cause of the lung disease silicosis, you should always wear an air-supplied hood when using it (see Photo 6, at right).
• Glass beads — These are used to remove carbon and surface residue. They clean, deburr and peen a surface and create a matte finish.
• Aluminum oxide — Sharp, hard and durable, this medium is used to frost glass, carve letters in stone and remove heavy contaminants.
• Silicon carbide — This quickly cuts granite, ceramics and other tough materials and is effective for cleaning, finishing and etching.
• Coal slag — Also known as Black Beauty, Black Diamond and Black Blast, it’s used to remove rust, scale and paint. It contains only 0.1
percent free silica, so it’s much safer to use than silica sand.
• Steel shot — This is typically used on heavy forgings or cast metals to clean, peen and remove heat-treated scale.
• Walnut shells — This soft medium is used for cleaning, polishing and deburring soft metals, fiberglass, plastic, wood and stone.
• Corncob — A biodegradable grit that is used to clean metal, wood, fiberglass and electric insulators, it won’t frost glass, pit aluminum or damage most surfaces.
All abrasive media are graded on mesh size (also referred to as sieve or screen size). As mesh size gets larger, the media become finer. To put this in everyday context, ground coffee (a fairly coarse substance) would have a mesh range of 16 to 40, whereas sugar (a much finer substance) has a typical mesh range of 50 to 80. Most types of media are available in different mesh sizes, and a local industrial-tool or sandblasting supplier can help you select the proper mesh size for your project.
In addition to mesh size, two other properties that affect the efficacy of any blast media are shape and hardness. Smooth, round particles will leave a peened finish, whereas sharp particles leave an etched or matte finish. And the harder the abrasive, the deeper it will penetrate and the faster it will work.
One last trait to consider in an abrasive media is its ability to be reused. Depending on what you’re stripping, some media can be screened and recycled. Check with your local supplier on reusing blast medium for your project.
If abrasive media can remove paint and rust, they can easily remove layers of skin or damage your eyes. It’s vital that you always wear the proper safety gear when working with any sandblasting unit. Wear elastic-cuffed coveralls to protect your arms and legs, heavy gloves for your hands and an air-supplied hood to protect your head as well as your respiratory system.
Some paint products retain their toxicity after they dry, and when they’re blasted off the workpiece and into the air, they become health hazards. Understand the nature of the substance you’re removing, and wear the proper respirator to protect yourself. A local sandblasting materials supplier can advise you on what’s best for your project.
Pay equal attention to your surroundings as you prepare your project. Abrasive media fly everywhere, so remove anything you can from your work area. Cover the floor with a tarp, and close any doors, windows or vents that would let airborne dust into your house.
The steps to properly operate a sandblasting unit are simple. Fill the hopper with abrasive media and set the air compressor to deliver the pressure required by your specific unit. (The Craftsman gun we used required 80 to 100 psi.) Just as in operating a paint gun,
position the sandblasting gun before the workpiece, press the trigger and move the spray pattern across the item, working in a straight line or in small circles (see Photo 5, at right). Shut off the spray after you’ve passed completely over the workpiece.
Trial and error will guide you in how far from the workpiece to position the nozzle for the best results. Work gently and don’t linger too long in one spot, as it is possible to destroy a delicate workpiece. Continue back and forth across the item until you’ve removed all of the desired material. Finally, plan to immediately coat or paint the workpiece to prevent flash rust from forming on the newly cleaned surface.
Photos by Scott Jacobson and Mike Berger and courtesy Campbell Hausfeld
National Institute for Occupational Safety and Health