Some one said, "Never take a finished bowl into the house from the shop on a sunny day. Sunlight causes scratches." I do have a video up on sanding, which needs to be upgraded, or 'new and improved'... So, first thing quality of your abrasives really makes a difference. I prefer the blue discs from VinceswoodNwonders. He used to send me samples of different abrasives to try out, and they worked, but I kept going back to the blue. They just cut faster and longer than any other abrasives I have tried. Not the cheapest, but best value.
Sanding speeds: Slower is better. I only use a drill for sanding. I keep drill speed to less than half of full trigger. Getting the abrasives to cut is all about traction, kind of like dragsters. They burn off their wheels to heat them up, but they have no traction so there is lots of smoke, but they are not moving. If you are spinning too fast, then you are sending out a lot of dust, but you are not really digging in and cutting. Coarser grits can go a bit faster than the finer grits.
Pressure: Minimal pressure, no more than the weight of the drill. Too hard and you generate more heat, and your drill wears out a lot faster.
Vary your scratch patterns... One problem with hand sanding is all of your scratch lines go in the same direction, so it is easy to have 80 grit scratches under 120 scratches and not see them. With the drills, you can use one edge of the pad or the other and get a different pattern, kind of ( or )... Some times I go both ways with one grit, some times I go one way with one grit, and the other way with the next grit. This is a standard practice on spindles, to sand with the spin, then stop and sand length wise.
Interface pads: Buy one good mandrill for your drill and a bunch of interface pads. Cheaper to replace the interface pads than the mandrill. Again, too much pressure and speed can cause the hooks to melt and wear out much more quickly. A firm pad (really love the one from Vince that has a 1/4 round edge rather than square) for grits up to 180 or 220, medium pad for up to 400, and soft pad after that. Firm pad with 120 cuts faster than 80 grit on a soft pad, and no, pushing with 80 grit on a soft pad does not get you into coves well, radius edge pad at a skewed angle will do it.
Grit progression: Rule of thumb is, when moving to the next higher grit, 1/2 the number, so from 80 you go 80 + 40 = 120 grit. Well, that kind of works, and I used to practice that. However, I have found that for the coarser grits, some times I start at 80 grit. Most of the time, now anyway, I start at 100 or 120. Even if I start at 80, I go to 100, 120, 150, 180, then 220, and 400. I seldom go beyond that. Main reason, is that if I leave a few 100 grit scratches, I can take them out with 120. If I leave 80 grit scratches, they take forever to remove with 120. 150 may be a bit redundant, but I am getting far more bowls that I can take from the shop into the house on a sunny days. The coarse grits 80 to 120, are for cutting and leveling, and removing tear out. They actually cut and remove stock and are good for rounding over edges. I use far more 120 discs than I do 220 discs. The grits from 150 to 220 or 320, are more for removing scratches from the previous grits. You can round over an edge with 320, but it still has a crisp feel to it, but not one that will cut you. In the 320+ grit range, you are mostly buffing out scratches. 220 and above will not remove tear out, no matter how long you sand. You may get it down to the point where finish will cover it, but not to the point where, if you sand up to 2000, it will be gone. Go back to 100 or 120 some times.
Mystery Scratches: Two keys here are good lighting and good glasses. You have to be able to see what you are doing. I prefer therapy type lamps more in the natural light spectrum, and some of the LED lights are now getting into this range rather than the straight white ones. We see better in natural sun light. If your eyes are going fuzzy, time to get good glasses. The standard cheap reader glasses work, but prescription glasses are far better. There are those who use the air hose to blow off the dust and loose or embedded abrasive particles. I never do this. In part because who wants to blow more dust all over the shop. I have used my hands to wipe off bowl dust forever, well, ever since the first time I used the air hose trick. I have NEVER felt any embedded pieces of grit in the wood. If I have a undercut rim on the bowl, there can be some in there which are gone with a wipe or two, and they do not hang around with the next grit up. Another benefit of wiping the bowls down with my hands is that, especially with the finer grits, it pushes dust into the scratches from the coarser grits and high lights them, especially if you are using a contrasting scratch sanding method. I have heard of some that will use mineral spirits or lacquer thinner to do this, but I don't want that on my bowls. Another consideration and source of mystery scratches can be the abrasives and the discs. I always use abrasive discs that are larger than the interface pads. A 3 inch disc on a 3 inch pad will never go on perfectly, so some of the hooks and edge of the interface pad will be on the wood and that leaves marks. Same with a stiff abrasive backing like some of the heavy abrasives used on drum sanders or belt sanders. That hard edge leaves 80 grit scratches. Another reason why I like the discs from Vince. They are on a mylar film which is very flexible. As said above, the open weave type abrasives cut very slow compared to the solid surface ones up till you get into the 180 and 220 grit range, then they cut about the same.
I have never been a fan of wet sanding, at least not for the grits up to 220 or so. Main reason is that the sludge can fill in tear out that you won't see till you get to the higher grits. Only exception is end grain forms that I want to warp as much as possible.....
Look up my sanding hood video as well. Just a hose gets some dust. A hose and big gulp type hood get a lot of the dust. My sanding hood I use gets all of the dust other than what falls on my hands. If you use a fan, it works better if it blows the dust away rather than tries to suck it away, unless you have a hood around the back of it to channel all the flow.
robo hippy