FINISHING SECRETS…No.2
My Five Rules of Sanding
The following was originally published as, “Sanding Savvy”, in the American Woodturner Magazine, Spring 1999 issue.
Sanding Is Important
Sanding is the second of the three steps in the turning process - turning, sanding, and finishing. These steps are not independent. A poorly turned surface will require more time sanding, a poorly sanded surface will result in a poor final finish, and sometimes the finish and sanding are done at the same time.
We are overwhelmed with information on turning tools and finishing products. Everyone wants to know about the latest tools and techniques; or the merits of various finishing products, and which ones will give them that elusive “perfect finish”. But nobody wants to hear about sanding, and very little is ever published about it. In fact, it is more important than the finishing because the final finish will only be as good as the surface under it.
Sandpaper, The Tool
We can’t discuss sanding without some knowledge about sandpaper (or cloth). The commonly used abrasives can be listed in the order of their hardness, sharpness, and durability: Ceramics, Aluminum Oxide, Silicone Carbide, and Garnet. Aluminum Oxide will remove wood faster than Garnet because it is harder and sharper, making a deep V-grooved scratch pattern on the surface, while the softer Garnet wears faster and leaves a pattern of shallow U-shaped grooves. It makes sense to start sanding with Aluminum Oxide and finish with Garnet. It also makes sense that the surface from garnet will appear to have a higher gloss because of the more rounded reflective areas in the surface scratches.
Don’t buy cheap sandpaper. The grit designations of sandpaper do not represent a uniform fixed particle size. Rather, they are a range of particle sizes; with the majority of them being the stated size. The number and size of the particles that are different from that designated depends on the equipment and the quality specifications of the abrasive manufacturer. A tighter grit specification costs more. Unless we have access to their product specifications, we have to rely on either price or experience to determine quality. Cheap sandpaper isn’t necessarily a bargain because it usually has a broader range of particle sizes within a designated grit size, and it is the big ones that we don’t want because they leave deep scratches.
I use and recommend the sanding medium, both sheet and discs, available from Industrial Abrasives. Their prices may be lower, but their quality is every bit as good as any of the major suppliers. They also have large 36” X 39” sheets of flannel backed Aluminum Oxide medium so we can cut our own sanding discs and really save some money.
The “Five Rules” of Sanding
Everything that I know about sanding can be reduced to “The Five Rules of Sanding”. These “5 Rules” will shorten the time between laying down the turning tool and realizing a museum quality finish. Since sanding can’t be ignored, we can try to make it an easier task.
Two prerequisites to sanding should be obvious. We can’t see in the dark, and we can’t work when we can’t breathe. Always use a bright incandescent light, the brighter the better, while sanding so that the scratch pattern on the wood surface is clearly visible. Aiming the light at a shallow angle to the wood will make them even more visible.
Fluorescent light has no shadows, and the surface texture can be almost invisible until you take the finished piece outside the shop lighting.
Always wear a dust mask while sanding, or better yet, use both a dust mask and a dust collector.
Rule 1 -
Sandpaper is a cutting tool, keep it sharp and keep it clean.
Throw it away when it gets dull. Don’t use a worn-out coarser grit as a substitute for a finer grit. Worn-out 120-grit is just that, and it cannot be used as a substitute for 220-grit.
Keep it clean.
The spaces between the grit particles are like the gullets of a saw blade. The grit can’t remove wood when these spaces are full. Clean both sheet and disc abrasives with a block of crepe rubber. (Yes, it works on sheet when we lay it on a flat surface and rub the rubber block across it). Some turners glue a piece of rubber to a block of wood that is then attached to their lathe. Blocks of natural colored crepe rubber are available through most of the woodworking catalogs, a $10.00 purchase that will last for years. Do not use colored rubber, white rubber, or any caulking materials because they contain dyes and silicones that can interfere with many finishes.
A small brass bristle brush, such as would be used for cleaning vegetables, is an effective cleaner for removing stubborn materials from the abrasive.
Remember those big grits we talked about??
They can be removed by dragging the sanding sheet across the edge of a piece of steel. This dressing of the sheet will break off those big particles that are sticking out above the surrounding grit. We can do the same thing to a disc with a brief spin against the same edge of a piece of steel, but have to be careful that we remove only the big ones and not the ones we want to keep.
Rule 2 -
Refine the shape, remove all tool damage and torn grain, and repair the surface with as coarse an abrasive as necessary to do the job - BEFORE moving through the finer grits...
The only purpose for all subsequent sanding with finer abrasives should be to refine and remove the scratch pattern made by the coarsest paper. Go straight to the 60-grit if there is torn grain or other tool damage on the surface, and don’t fool around with anything finer. It makes no sense to brag about never using anything coarser than 180-grit, and then spend two hours to do the same job that could have done in ten minutes with 60-grit. The finer grit can be used, and for a shorter time, when we have improved our tool handling skills during the turning phase of our work.
Rule 3 -
Sand through all of the progressively finer grits in as fine an increment as available.
Don't skip any grits, and don’t quit before 320.
It is much faster to use all of the abrasive grits in the smallest increments possible, than to make large jumps in grit size. This is particularly true at the coarse end of the scale. I assure you that it is quicker to go from a 100-grit to a 320-grit in several steps than it is to go directly from the 100 to 320 grit.
For hand sanding, I usually start with 60 or 80, and proceed through 100, 120, 150, 180, 220, 280, 320, and stop at 400. Did I miss any? Several grits can be eliminated while using the more aggressive power sanding, and I typically use 60, 100, 150, 180, 240, 320, and 400 for power sanding. I prefer to use the 2" discs because there is less torque and they are easier to control.
Rule 4 -
Remove all of the scratches from the previous grit, and all of the dust from the present grit before going to the next finer grit.
This step is faster when Rule 3 is also observed. If we leave the scratches from the previous grit, we will be making it more difficult to remove them with the next finer grit. If we don’t remove the trash from the present grit we will still be abrading the wood surface with it when we go to the next finer grit.
Rule 5 -
Slow is good, and slower is even better.
Patience is a virtue.
There is a universal tendency to sand too fast. There is an ancient rule that we should be sanding at twice the speed that we used for the turning. It is even worse for power sanding because we think we can get it over with faster by running everything wide open.
Heat is the enemy. Keep the sanding medium cool by sanding slowly. And, the slower moving abrasive will remove more material than one that quickly “skates” across the wood surface.
Don’t spin the piece in the lathe so fast that the paper gets hot. If it burns your fingers, it is also burning the wood surface, and casehardening rather than cutting it. We have all experienced the situation where the sandpaper quit cutting, and it took a coarser grade to break through the surface glaze, only to have the same problem when we used the finer sandpaper. When this happens, slow down. I typically sand with a maximum lathe speed of 250 RPM for most work.
It is very easy to generate too much heat when power sanding because we have no direct touch with the abrasive. Heat will clog the disc, and destroy either the cloth backing or the sponge rubber on the arbor. Just because we have an electric drill with a top speed of 2400 RPM doesn’t mean that wide-open is the best speed for sanding.
The slower speed will remove more wood faster, the disc is easier to control, there is very little airborne dust, and the sanding discs will last longer. I use a 3/8” Black and Decker Magnum drill for power sanding wherever it will reach because it has an excellent trigger control and it will run continuously at less than 150 RPM. I use a Milwaukee (same as Sioux) angle drill for those areas that I can’t get at with the old B&D. And I have a 1200RPM air powered right-angle drill for the insides of things that nothing else will fit into.
There are many woodturners who will argue that running the disc sander at a high speed while the work is spinning slowly in the lathe is the best way to go, and there are still some who believe that running everything wide open is the best way to go. To them I say, try running both at a lower speed and see if the sanding isn’t just as fast and easier to control. Two other benefits will be less airborne dust and the sanding discs will last longer.
It is most difficult to convince someone to try sanding at slower speeds. Once convinced to try it, they usually become advocates of sanding SLOW.
