Extraction and protection when dry-sanding
George Di Scala, technical service engineer of 3M Australia’s automotive aftermarket division, writes about the benefits of dry sanding over wet sanding.
When 3M introduced the patented WetorDry Sand paper way back in 1921, it was seen as an innovation in the fledgling automotive manufacture and repair industry.
To some extent this is still seen as the best practice in a number of countries including some repairers in Australia. However, with advances in tools and abrasive materials and, more importantly, changes in paint technology such as the introduction of water-based technology, wet sanding is becoming a less suitable option.
Moving to a dry sanding process has a number of benefits, including, increasing productivity, a cleaner shop floor and generally a better overall finish is obtained. However, as the dust is no longer being captured by water, an extraction system is required, whether that is a central vacuum or self generating system, will depend on the shop fit out or process. Dust extraction also aids in extending the life of the disc by increasing the time to loading on the disc, which is also supported by anti-loading coatings applied on the disc (pictures a, b, c).
How dust is created
Essentially, the grit size will change the size and number of dust particles generated. If you consider a P180 grit disc or finer, the particles produced during the sanding process are so fine and small that they will readily become airborne, whereas particles created during operations with P80 grit disc are much larger and tend to remain on a flat surface, or fall to ground when sanding on a vertical surface. They are less likely to become airborne.
To aid in extraction, the abrasive disc needs to be designed to allow dust extraction. While this may look simple, there is a fair amount of science that goes into designing a disc for optimum extraction. Properties such as the size of the orbit on the sander, grit size, particle size, hole size, greatly affect the effectiveness of dust extraction. Over time at 3M, this has evolved from a six hole system to seven, then fifteen, and now a multi-hole configuration.
Why has 3M seen the need to change the hole configuration? Increasing the amount of holes helps improve the dust extraction, while not significantly between six and seven holes, there is a noticeable difference once you move to a fifteen-hole system. The multi-hole construction is altogether different, in that the holes are smaller, however designed in a pattern to optimise dust extraction.
You still need to wear a mask
The effectiveness of dust extraction varies depending on a number of factors. Consequently, there is still a need for some personal protection, namely in the form of a respirator. While the fine nasal hairs in our nose are great for filtering out large dust particles, they will not provide sufficient protection against the fine particles generated during the sanding process. If you do a walk around of all the repair shops in Australia, you will struggle to find a technician wearing a respirator while sanding. Why? The most common reason given is that wearing a respirator is uncomfortable and too hot to be wearing all day.
WorkSafe Western Australia Commission’s Code of Practice for Spray Painting states the following for sanding automotive paints: “Grinding and sanding of painted surfaces generates dust that can expose workers to hazardous concentrations of toxic substances such as lead, antimony, tributyltin oxide, nickel and hexavalent chromium. Sanding of polyurethane or epoxy paint that is not fully cured generates dust particles containing unreacted hardeners.”
One study, Lead and Methylene Chloride Exposure Among Automotive repair technicians, found that those who remove paint at large repainting facilities are more likely to be overexposed to lead than those who apply it, with effects such as affecting fertility. Hexavalent chromium is a known lung carcinogen. These are just a couple of reasons why respiratory protection should be worn when dry sanding.
So what respirator is generally required in the sanding process? Look for a P1 rated mask meeting Australian Standard, AS/NZS1716:2003. These masks have been designed to filter mechanically generated particles, such as those from sanding. 3M Australia has a number of P1 rated masks that will be suitable for this process. 3M Australia recommends the 9312 (picture 3), however other products are available for the budget-conscious operator. The 9312 mask offers the wearer a diaphragm which allows breathing vapour to vent out, so that operator remains comfortable for longer periods of time.
More information from at www.3m.com.au/aad.
