Photo courtesy of Mike Grim's Truck Wreck Specialists (www.mikegrims.com)

If the old adage is true, that "the devil is in the details," then preparation work may be Old Scratch's birthplace. Few tasks offer maximum results without a maximum amount of meticulous advance effort.

Areas to be welded need to be free of coatings, grease, rust, scale and other contaminants to ensure solid, long-lasting bonds. Builders of new equipment are fortunate, in that they most often need only to wire brush or grind joining surfaces. On the other hand, rebuilders and restorers, including shops that perform mold repair, have more prep to perform and frequently rely on sandblasting to remove stubborn material and oxidation.

Blast cleaning, generally, is a fast means to clean complex surfaces without the use of chemical stripping agents. With experience and finesse, sandblasting can handle most power abrasive tasks in a shop; however, substrates other than steel often don't tolerate such aggressive surface preparation as well. Many times it's easy to blow holes right through the article being cleaned. Wet chemical strippers can remove coatings, but they generate hazardous wastes that take time to dispose of properly.

Other media
Before putting away the blasting equipment, however, one should consider an alternate load in the media hopper.

Plastic abrasives, for example, deliver a high stripping rate and consistent performance. Plastic media is considered too soft for most rust removal, but it is good for applications such as removing paint, mold cleaning, deflashing and deburring.

There are three types of plastic media commonly used:

• Acrylic is the longest lasting media presently on the market. Termed a multipurpose media by users, acrylic is very gentle on substrates and engineered for stripping the most sensitive surfaces while providing an effective stripping rate.
• Melamine is engineered for stripping the most difficult surfaces while providing an effective stripping rate, and can be used as a replacement for glass beads and other harsh abrasives.
• Urea is the most widely used plastic media, particularly in automotive work, because of its low cost and because it can be recycled for use several times, as compared to chemical stripping. Typically used for less sensitive applications, urea is formulated to meet an increased level of stripping performance where stripping speed outweighs other considerations.

All three plastic media are available in mesh sizes from 12-16 to 60-80 (12 mesh is equal to 1.7 mm, while 60 mesh equals .25 mm). Applications range from aircraft and aircraft engine components, to metal diecastings and soft metals like aluminum.

Aluminum oxides
Aluminum oxide grit powder has a wide variety of cleaning applications, from engine heads, valves, pistons and turbine blades in the aircraft industry to lettering in monument and marker inscriptions. Harder than other commonly used blasting materials, aluminum oxide penetrates and cuts even the hardest metals and sintered carbide, and can be recycled many times. The media can be expensive (though less so than plastic), particularly if there aren't means for recovery, but it can strip thick metal, such as a vehicle frame, in short time.

According to one supplier of aluminum oxide abrasive, virgin brown aluminum oxide provides optimal performance, when compared to reprocessed product. Virgin, brown aluminum oxide contains less than 1.5% free silica and is therefore safer to use than sand. The grit size, ranging from 12 to 220, is consistent and cuts much faster than sand, leaving a smoother surface.

White aluminum oxide is safer to use than sand because it contains less than 0.2% free silica. As a blasting media, white aluminum oxide is 99.5% ultra pure, with twice as many particles per pound than other metallic media. Fast cutting action minimizes damage to thin materials by eliminating surface stresses caused by heavier, slower-cutting media blasting grits. Grit size is consistent and cuts much faster than other sand blasting media, leaving a smoother surface.

Glass and...nuts?
The angular particles in crushed glass grit allow for aggressive surface profiling and removal of coatings such as epoxy, paint, alkyds, vinyl, polyurea, coal tar and elastomers. Crushed glass grit is produced from recycled bottle glass, with no free silica. It is non-toxic and inert, and contains no heavy metals.

With thin metals, blasting media that generate heat, such as sand or aluminum oxide, should be avoided. However, there are alternatives. Blasting media made from walnut shells is considered a "soft abrasive," yet is extremely durable, and sufficiently angular and multi-faceted to strip light to moderate rust. Common blast cleaning applications with walnut shell grit include stripping auto and truck panels and cleaning delicate molds.

Equipment
Most blasting machines can handle any kind of a dry blast media, according to Steve Schneider, sales manager at Kramer Industries (sschneider@kramerindustriesonline.com), a supplier of abrasives and equipment for finishing a wide range of materials. He adds that some cabinet machines might need modificationfor lighter abrasives like plastics, to keep the media flowing from the hopper to the blast line. And, says Schneider, nozzle size is important. Never use a small nozzle with large abrasive.

Both sand and crushed glass grit are consumable medias — they are good for one use, then they must be discarded. Other types of media can be used in cabinets and booths equipped with systems that recover all of the particulates and separate the grit from removed paints and coatings. Re-use of such media will lower materials costs in operations where stripping is frequently performed.

Schneider also notes that users often ask why the abrasive life seems to be unreasonably short. There are 3 reasons this may be happening:

1. The operator is blasting too close to the work surface. If operating too close to the surface, the abrasive bouncing off the surface is colliding with the abrasive from the gun. The blast gun should be a minimum of 6-in. from the surface, or, if using a direct pressure machine, the minimum is one foot.
2. A small parts such as a washer or other foreign object may have been sucked up into the abrasive hose. Abrasive colliding with this obstruction will get pulverized. Check hose lines regularly.
3. The blast pressure may be too high. Most abrasives should not be blasted at pressures above 100 psi. Glass beads will deteriorate at pressures above 80 psi.

The addition of blast equipment to a shop, particularly one involved in overhaul and repair, will help quickly identify the extent of weld work needed on a job, as well as providing a clean surface for welding. If floor space doesn't permit room for blasting, a reliable stripping service could be a valuable asset in the company Rolodex.