It seems the course grits cause problems leaving sanding scratches that are almost impossible to remove.
When sanding and polish metals for microscope examination there is a thing called subsurface deformation. (In the '70s I did prep, etching, and examination of grain growth in aluminum alloys on nuclear reactor elements.) A scratch from a single coarse grit can cause damage below the bottom of the groove that is revealed far later in the process. The cure is to removed more than needed with each finer grit.
I always suspected a similar thing happens with wood under certain conditions, big coarse grains digging a trough and deforming fibers deeper than the bottom of the groove. I imaging this might be worse with certain woods and with an excess of pressure/heat.
I despise circular sanding scratches. My sanding:
When sanding spindles by moving the paper against the rotating wood, I stop the lathe after every grit and sand perpendicular to the circular scratches (generally with the grain) until they are removed. My theory is if I can't take out 400 grit scratches with 400 paper, I'll never get them out with 600 grit paper. (It's worse with coarser grits.) Primary rule when sanding with the lathe spinning: spin very slowly. Other things: avoid excessive pressure and especially heat. As mentioned, good lights and good vision are necessary, not just useful. I generally sand spindles with the lathe in reverse with a mini-gulp positioned behind the work to suck up the dust.
When sanding face work I generally don't, much. I turn as cleanly as possible being absolutely sure to remove any tearout. Then I use hand-held, generally curved, usually small cabinet scrapers to remove remaining tool marks and ripples such as in the middle of the inside bottom of the bowl that I didn't cut cleanly enough. This allows starting with a much finer grit. Instead of 80 grit, I usually start with 400 grit or 320, occasionally 220. This actually is quicker than sanding through the grits for most things. People ask me how in the world did it get the inside bottom of the bowl that smooth. That's how.
I dislike power sanding because of the dust. When it makes sense to power sand, I've switched from the rotary sander (angle drill) to a 2" or 1" sanding pad on the tiny Grex random-orbital-sander. Rudy Lopez recommended this at the TAW symposium this year. This thing is air operated and is amazing. With the trigger pressed lightly the speed is very slow. The random orbital motion minimizes sweeping scratches with even the coarser grits where needed. This method does require a good air source.
As mentioned, good lights and good vision is mandatory. I'm convinced that some of the horrible circular sanding scratches some people leave in the work are simply due to their vision problems. This is a shame on otherwise beautiful work.
I prefer small, bright light sources on swing-arm or gooseneck fixtures at the lathe both for turning and for inspection. A light positioned at a glancing angle (tangent) to the surface is best since the bottom of a scratch will be in the shadow and easier to see. The "point source" lights are also, in my opinion, best for turning since they let me see and judge the compound curves as I'm forming the surface. Small turning defects such as unwanted inflection points, ripples, and curves other that what I had in my mind are far easier to see and to fix. In my opinion broad, diffuse lighting such as indirect lighting or multiple long fluorescent fixtures high on a ceiling above the lathe are the worst for seeing scratches and defects since the lighting is too even. Unfortunately, many shops are lighted this way. In this case, one or two bright movable lights (or a bright flashlight) will go for towards the perfect finish.
During and after sanding I occasionally wipe the surface down with a fast-drying liquid. With the good lights, this is amazing at highlighting the scratches. I generally use naptha, sometimes alcohol to raise the grain (very useful when sanding). I'm sure many people have watched Jimmy Clewes use alcohol for this and ignite it with a cigarette lighter to dry it quickly. I don't do this. I don't have central fire extinguisher system installed in my shop.
JKJ
