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Sanding, Part 1

Introduction


All tool marks must be removed from a piece for it to be considered anything but an effort by a novice.  And we remove tool marks by sanding.  But sandpaper is scratchy stuff, and the course sandpaper required to remove the tool marks will create scratches that are just as ugly.  So, after the tool marks are gone, we have to get rid of the sanding scratches.


This is done in a series of steps using progressively finer grades of sandpaper.  The objective at a particular step is to remove the scratches left by the previous step.  When the scratches become so fine that they are invisible, we are done. This can be a lengthy procedure.  It is not at all unusual for a turner to spend more time sanding a piece than it took to do the turning.  


A worthy objective is to develop the skills in using the tools so that fewer and smaller tool marks are produced.  This allows a finer grade sandpaper to be used in the initial step so that the first batch of sanding scratches are smaller and more easily eliminated.


Sometimes, in spite of your best efforts, the wood will not cut cleanly.  This gives rise to tear-out where wood fibers have been pulled or split off the surface.  Also, there may be regions where the grain appears to be scuffed or smeared.  These effects add to the burden of sanding because they must be removed.   


Compared to using a gouge or a skew, sanding is a rather simple proposition.  However, when I first started sanding turnings, I found it to be very frustrating because I would often run into scratches or a blemish that simply would not go away, and I didn’t know why.  


In this article, after the basics, I explain what causes a lot of problems and how to deal with them when they arise.  Sanding is still not high on my list of fun things to do, but I’m no longer afraid of it. The time it takes to sand a piece is now just a fraction of what it used to be.


Note: I use the term “sandpaper” even though the backing may be cloth or some material other than paper, and the abrasive may be something other than sand.  


Basic Concepts and General Observations


Safety Note:  You will be close to the wood when sanding, and close to the chuck (if one is being used) as well.  Sandpaper can slip or grab. Experience indicates that you are more likely to make contact with the chuck jaws when sanding than at any other time. Be careful.


Sandpaper. The quality of the sandpaper you use will have a big impact on the efficiency of the sanding process and probably the end result.  Avoid the stuff at the big box stores, home centers, and other non-specialty places; it’s hardly worth hauling home.  The difference in performance between a good sandpaper and one that is sorry is dramatic.  


A variety of good-quality abrasives are available from the sources given at the end of Part 3 of this article.  Good sandpaper is not cheap. The lowest price does not guarantee the least overall cost because the more expensive materials typically last longer.  Try samples of several varieties and, over time, you can decide which you prefer.


The grit.  The courseness of abrasives is specified by a number, the grit, with lower numbers referring to courser material.  The grits that a typical turner might use on a regular basis are 100, 120, 150, 180, 220,  240,  320, and 400.  You may find a need for 80 grit, but hopefully, rarely.  Pen turners typically work with special abrasives with grits much, much higher than 400.  


The turning tools.  The time you have to devote to sanding can be reduced if you constantly strive to get the best possible surface “off the tool.”  First and foremost, the tools should be sharp.  A dull tool will never leave a good surface.  


Beyond this, the choice of tool and the type of cut employed has a major impact on the quality of the surface left behind. On spindles, a skew gives the best-possible surface. On other pieces, a shearing cut with a gouge or scraper is usually the best option for the final cut.  


Removing tool marks.  The first objective is to get rid of all tool marks and ripples in the surface.  Begin sanding with the coarsest grit (lowest number) the surface requires.  This is a judgement call. If the surface is unusually rough, an abrasive as coarse as 80-grit may be appropriate.  However, having to use 80-grit paper routinely usually indicates that a problem exists either in using or sharpening the tools.


Resist the temptation to start the sequence at too high a grit. Sometimes a surface will appear flawless right off the tool, but when you begin sanding at 180 or 220, the blemishes may spring forth almost as if they had been produced by the sanding itself.  In this case, drop back to a lower grit.


Sanding through the grits.   Because the abrasive removes wood by essentially scratching it away, the surface will be covered with sanding scratches after it is leveled.  The objective from this point on is simply to get rid of the scratches produced by the initial step.  


If 100 grit is used first, the next step should be 150.  This is to replace the scratches left by the 100 grit with scratches from the 150.  After 150, proceed to 220 or 240 and remove the scratches left by the 150.  Next, use 320, and then 400.  Most turners stop at 400.  

Many turners recommend hand sanding for the last step of a particular grit.  That is, stop the lathe and sand by hand, with the grain.  Finally, wipe all sanding dust (and possibly loose grit) off the piece, or blow it off with compressed air.  Then proceed with the next higher grit.


You will not save time by skipping a grit because additional time will be required for the finer grit to do the job of the one you skipped. Skipping a grit also makes it more likely that you will leave some of the scratches from the previous grit, and these leftovers will appear in the final, finished surface.  


Sanding speed.  Running the lathe (or drill, if power sanding) at high speed greatly increases the amount of heat produced while sanding.  The heat can make holding the sandpaper an uncomfortable proposition, and it can cause sanding checks to appear on the surface.  These are small cracks that sometimes develop when the wood is overheated.


But more to the point, sanding at high RPM does not necessarily result in faster wood removal.  High speeds can cause the abrasive to skim or skid across the surface without digging in to cut the wood fibers.  The result is frequently a shiny, burnished, hardened surface with embedded scratches.  Your only option at this point is to reduce the speed and drop back to a lower grit to cut through the burnished layer.  Only then can the embedded scratches be removed.


I’ve found that 800 RPM works well for a bowl 8” in diameter when sanding with 100 or 150-grit paper.  For the higher grits, I may speed the lathe up to 1,000 RPM.  This is not a hard and fast rule.


Do a bit of experimenting with different speeds and see what works best for you.  Slow the lathe way down (to perhaps 300 RPM) and see how fast sanding dust is produced.  Then increase the speed and see if the rate of dust production (and wood removal) increases.  It may or may not, but the amount of heat generated surely will.


Opinions and preferences vary considerably.  Some turners are advocates of  “slow sanding” where the lathe is slowed to a crawl. Others prefer to turn the RPM way up, to speeds even higher than what are used for turning.  


Dull sandpaper.  In use, sandpaper gradually gets dull.  It looses its tooth – that scratchy characteristic that enables it to bite into the wood.  


Symptoms of dull sandpaper:  (1) the rate of wood removal decreases;  (2) more pressure is required to get it to cut, which increases heat generation;  (3) it becomes rather ineffective in removing scratches;  (4)  it tends to produce a slick-looking surface with embedded scratches.


Sanding should not produce a shiny surface; this is burnishing, typically caused by using dull paper or an RPM that’s too high. However, as you work toward the higher grits, the surface will develop a soft sheen.  


Sometimes I encounter stubborn scratchs that seem, at first, to demand falling back to a lower grit.  However, in many cases, simply switching from a dull paper to one of the same grit that is new and sharp will make them go away.


This is an obvious point, but I’ll mention it anyway.  Dull 150-grit sandpaper, for example, does not equate to 180 or a higher grit.  In the words of one experienced turner, “Dull sandpaper is just dull sandpaper.”  Throw it out.


Sanding should produce dust.  This is true, even at the higher grits.  If no dust is being produced, the sandpaper is probably dull or clogged and should be discarded.  Obviously, much less dust will be produced at the higher grits, but the principle still holds true.


This applies equally to sanding done to level a finish such as lacquer or polyurethane.  A lack of dust indicates the abrasive is not cutting, and the result is not likely to be satisfactory.  The probable cause of the lack of dust in this case  is that the finish is not fully cured.


Clearing the dust.  For the sanding to continue in an efficient manner, the dust must be ejected from between the abrasive and the surface.  With dry wood that is free of rosin or gum, this occurs naturally for grits up through about 180.  The dust does not clear as easily for grits 220 and higher.  


Dust trapped between the abrasive and the surface being sanded significantly reduces the efficiency of the sanding process.  Further, it tends to clog the paper, which renders it useless.  Therefore, you must stop frequently and clear the dust, either by brushing it away or blowing it off with compressed air.  


With wood that contains considerable moisture or which tends to be gummy, the abrasive may develop corns.  These are caked-on globs of dust and debris that are not removable in any practical sense, and of course, they ruin the abrasive.  


Getting corns on the abrasive is a problem I often encounter when I attempt to sand a surface that has been coated with lacquer sanding sealer.  Unless the sealer has fully cured, which takes several days, getting the corns is almost a certainty.  


A magic trick.  Here’s a tip that in itself is worth the price of admission to this website.  I discovered it all by myself, and then learned that other turners have been using it for years.  Nobody told me . . .


The trick is to use compressed air to forcibly blow the dust off the surface being sanded at the same time the sanding is being done. That is, hold the sandpaper with one hand and the air gun with the other, and then sand and blow at the same time.  This works wonders with grits 180 and above.

Not only does the sanding go faster, but the sandpaper is far less likely to load up and clog. Further, it extends the useful life of the sandpaper. The only drawback is that it scatters dust all over, so you need a good dust collection system.  


Holding the paper.  A rectangular swatch of sandpaper can be folded over into halves or thirds to make a pad.  Such a pad provides additional thermal insulation between the surface and your fingers.  Also, it will be somewhat stiffer which may be helpful when leveling a surface.  


The corners of the pad can catch on small voids so it’s important to keep the leading edge lifted above the surface.  This is more likely to happen while sanding the inside of a bowl.  Keep the abrasive moving to avoid getting flat spots and to help clear the dust.


I sometimes use a two-handed approach to holding the paper because it gives me better control.  One hand keeps the paper stationary while the other applies the pressure.  If you do this, be careful about reaching across the top of the workpiece or chuck so you don’t become entangled.  


Safety note:  Be wary of sharp edges or corners such as may appear at the rim of a bowl.  In principle, these should be relieved with turning tools before you begin sanding, but the sanding process itself can generate a sharp edge that will inflict a painful cut or burn if you happen to bump against it.  Stay alert.


Up next:  Power sanding.


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