February 14, 2012

 

Replace Hand Sanding with Power Sanding


Bodyshop Business (www.bodyshopbusiness.com) has a new, updated website that includes links to articles that were published years ago in their print magazine. Here's a portion of one of their sanding related articles from March 2010 titled "Why You Should Replace Hand Sanding with Power Sanding":
Removing Material

This is the portion of the work that’s seldom done by hand by anyone. All you vo-tech grads probably still have the long board (a.k.a. the idiot stick) that you started school with and maybe even drag across a troublesome panel once or twice a year, but in large part everyone recognizes that removing old finishes or shaping body filler is a job for power tools. This rough work can be accomplished quickly by using a:

• Grinder. While the 24 or 36 grit 7-inch grinding disc will rip material off the surface fast, it also generates lots of heat. Any tool spinning at 3,000 to 5,000 rpm gets the sandpaper and the surface really hot. When you get the old paint or new body filler hot, it melts and clogs the expensive grinding disc. Let’s not forget also that it takes a manly man to shape filler with any kind of finesse when the tool is heavy and whirling at 5,000 rpm.

The other part of the equation is that once the material is ripped off with the grinder, the tech now has to fill and smooth those 24 grit scratches, adding more labor time to the repair. A better choice to strip paint and shave filler with 36, 40, 50, 60 or 80 grit paper might be a hog sander.

• 8-inch hog sander. This tool operates at a much lower rpm (900 to 1,000) and therefore runs much cooler. Because it spins slower, you can sand off the material with a paper disc rather than cloth. Materials are much cheaper. Whether this style of tool runs in a rotary (straight circle) or a rosette (offset circle) pattern, it removes the old finish or the body filler quickly and with lots of control.

To get the maximum benefit, make sure to hold the sander flat against the surface rather than up on the edge like a grinding disc. There are still some techs who think that to keep a long panel straight, they need a tool with a longer pad than the 8-inch round ones found on this style of sander. They prefer to use a file board.

• File board. With a 2-3/4-inch by 16-1/2-inch pad, this powered “idiot stick” is the staple of the industry. If you’re old enough, you remember when this tool was equipped with a 14-inch metal file rather than a sanding pad, and metal men used them to power-file down high spots in the steel and shave body solder (lead). Hence the name “file board.” Today, virtually all file boards are equipped with sanding pads. These tools can run in a straight line (back and forth) and be powered by a single piston with two ringed ends or by two pistons, one to push the pad forward and the other to push it back. Or they can run with an orbital wiggle (jitterbug) pattern.

Both styles have their advocates and strengths. In either case, it’s a great way to keep a long panel straight and chop off unwanted material quickly. Keeping piston-driven air files well lubricated will minimize the stalling of the pad. When the air pressure is equal on both ends of the stroke, the tool won’t start up when the trigger is depressed the next time. Many techs slam the end of the sanding pad to get the tool started back up, but keeping the cylinders well oiled is the key. And just a light push on one end or the other will start the tool back up smoothly.

Shaping the Surface

Shaping sanders are tools designed to prepare surfaces for paint. Power sanders do this rapidly, increasing the adhesion of the new coatings and featheredging the chips in the old finish. A smaller version of a file board sander with a straight line action could also work well to block the surface flat.

Prior to 1957, this type of sanding was often done by an orbital sander with a 3-inch by 8-inch or 3-2/3-inch by 9-inch pad called a jitterbug due to its wobbling action. Orbital sanders with various size pads are still useful today. Several sander manufacturers offer square or rectangular pads on a drive motor with a 3/16-inch orbit. In fact, most tools in this category (based on my arbitrary definitions) have a 3/16-inch or larger (3/8-inch) stroke – fast and aggressive enough to quickly shape and prepare the surface.

Just a reminder about how important thorough sanding is to ensure adhesion of the new coatings. It’s possible to double the contact of the surface area by sanding it well. How can that be? Imagine a distance on a flat surface from point A to point B. Now sand that same area and the distance from A to B (the contact area for the new coating), including the surface on both sides of the scratch (trench). Down one side of the scratch, up the other, down the next, up the other side – you get the idea. It will provide twice the surface to stick the new finish to.

A rotary or orbital tool runs the pad in a circle. A random orbit sander runs the pad in two circles, one inside the other. In 1957, the National Detroit Company patented the first random orbit sander widely used in refinish. In an effort to describe the circle-within-a-circle pattern, they called it “Dual Action.” It has become so pervasive in the market that its brand name has become a generic term. So when you say “D/A,” you likely mean a random orbit sander, now made by dozens of quality companies – kind of like when you say Kleenex and mean facial tissue, originally made by the Kimberly-Clark Company, or when you say Bondo® (a trademark of 3M Company) and mean polyester body filler, originally made and successfully marketed by the Bondo Corporation (now 3M Company). Or, if you’re in the South and say Coke, you really mean a carbonated soda made by any one of dozens of brands!

In this middle category of power sanders, most random orbit tools use either a 3/8-inch (very aggressive) or 3/16-inch (less aggressive but still fast) offset. The size of the offset determines how far the pad travels in each rotation. Bigger offsets cover more area but cause the tool to vibrate more.
In addition, some of these tools have a built-in wobble in the pad that interrupts contact with the work surface, and with the pad always changing planes, the sandpaper doesn’t load up as quickly and the pad runs cooler. What sort of wobble? Take a dinner plate, set it on its edge and spin it like a top. As the plate slows down, it will wobble through the last few rotations just before it stops and lays flat again. The pad works the same way, lifting its edges at the outside, allowing the sanding residue to be cast out from under the pad.

Sandpaper grits that would be used in this shaping tool segment run from 80 to about 400. By the way, there’s a two-grit jump from hand sanding to machine sanding. For example, if the tech hand sands with 400 grit, he or she can get the same scratch by using 320 grit on a random orbit power sander. The power tool moves much faster. As another example, if the tech runs 180 grit on the D/A, he or she will need to move up two grits to 240 grit to get the same scratch by hand. In many shops, it’s common to see the painter run a random orbit sander with 320 grit and then hand sand the same area with 400. Hello, it’s the same scratch! So stop hand sanding, or at least move to 500 or 600 grit to see a finer result.

Race to the Finish

This is the point at which many techs pick up the rubber block or foam sanding pad, believing that the sanding must be done by hand to get quality results. In addition to a change in attitude, they need a very smooth-running sander! Final block sanding can be accomplished with super smooth versions of almost every tool we’ve described so far.

A straight line sander with a small stroke and velvety action is how craftsmen achieve the high-gloss finish on fine wooden furniture. Orbital or random orbital tools at this level will have a smaller offset, typically 3/32 inches, half or less than the sanders previously listed. These tools will remain in contact with the work surface (no pad wobble) and run in a nice, silky stroke. The three main offsets for random orbit sanders get smaller by half each time. The largest, most aggressive offset is 3/8 inches (6/16), with the next being half that or 3/16 inches (6/32). At the final finish level, the typical offset is a short and smooth 3/32 inches.

In my experience, small-stroke, smooth-running sanders are the type missing from many techs’ toolboxes. The reason, I believe, is techs’ fear that these sanders won’t run smoothly enough and be controllable enough to ensure the work will turn out perfect. Also, these tools are expensive, and power-sanding final detail work requires a change in procedure that many techs are uncomfortable with. In the end, far too many folks pick up a rubber block and scrub away at the surface at 67 cents per minute ($40/hour) labor time. If you buy the idea that more work could be done at the same or better quality, then you’re in the market for a super smooth power sander.

Pads: 5-Inch or 6-Inch?

Two common choices of pads are 5- and 6-inch diameter models. There are still some painters who use 5-inch pads, arguing that they can get closer to the edges and moldings with smaller ones. If you’re one of those folks, please do this little test: Take a 5-inch sanding disc and a 6-inch disc, and lay the 5-inch disc as close to the corner of a piece of notebook paper as you can while still keeping the whole sanding disc on the paper. You can’t get any closer to any car molding than this. Draw along the edge of the disc. You’ll have a curved line about a half-inch back from the corner. “See how close I can get?” say the 5-inch disc advocates.

Now take the 6-inch disc and slide it as close to the edge of the notebook paper as possible and draw a line on its curved edge. Guess what? The line from the 6-inch disc is within 1/8 inches of the 5-inch line. My point is that the 6-inch disc will get just as close to edges and moldings as the 5-inch one, but it has 44 percent more surface area. The tech would sand 44 percent longer between disc changes and the cost is only about 35 percent more per disc – a bargain and a great production aid, too! The final finish power sanding I’m describing will require sandpaper grits from about 400 up to 2,000. Remember that two grit jump: 1,200 on a D/A is about the same scratch as 2,000 by hand.

All Wet

Much of the work at this level is the sanding required on the clearcoat prior to a buff and polish. As much as 90 percent of this work is done with dry sandpaper, but I contend there is still a place for power wet sanding. Extra fine grits of dry sandpaper work so well because the abrasive manufacturers have done wonders with the lubricants they build into the latest versions of today’s papers.

The very first lubricant added to dry sandpaper was zinc stearate. These sandpapers were white instead of brown and were advertised as no-load or free cutting. The residue from sanding is correctly called “swarf.” It’s composed of both the sandpaper disintegrating and the dust from whatever you’re removing. Lubricants in sandpaper help to slide the swarf out of the way. Current dry-sand fine-finish papers use an even more sophisticated lubricant to prevent the paper from loading.

Wet sanding has always been useful for making sandpaper last longer as the water washes the swarf completely out of the way. The water also acts as a lubricant, cools the surface, extends paper life by three to five times and, when final sanding clears, allows for a bigger margin of error. The error we’re talking about is the clear resin that re-forms into a ball from the sander’s heat and gets caught under the pad.

I contend that painters who still hand-sand do so because they tried power sanding and dragged a ball of clear across the finish and left a 220 grit-sized scratch they couldn’t polish out. Fine finish wet sanders either pull water up to the surface by suction (caused by the rotation of the pad) or force-feed the water by connecting the sander to a faucet. Because the water will flush the surface completely, the chance of dragging a lump of melted clear across the finish is minimized.

The downside of wet sanding is that it’s messy, it can rust the tool’s components and it’s potentially dangerous when the floor gets slippery. One solution is to wet-sand using a spray bottle of water to flush the swarf away. If you prefer to dry sand, make sure to use plenty of interface pads, which act like a cushion and soften any contact between pad and work surface. Vacuum attachments will also serve to collect any hunks of swarf before they can damage the clearcoats – and keep the shop much cleaner too.

Some great information, huh? If you'd like to read the complete article, here's the link: "Why You Should Replace Hand Sanding with Power Sanding"




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