Others with more experience will, no doubt, add to this. You've seen that sanding bowls is much more difficult than sanding spindles, because you are sanding side grain and end grain both. For me, the key is to be more patient than I am normally. I need to progress through all the grits and make sure that I remove the previous grit's scratches before moving on. Wiping with spirits before moving on can reveal scratches that I've missed. Good lighting is key. Finally, there is a saying about using sandpaper like someone else is paying for it. I struggle with that, since I'm the one paying, but it makes sense to discard sandpaper before it's dull and causing scratches rather than removing them.

Grant Wilkinson wrote on Oct 27^th, 2020 at 8:03am:
Glenn Jacobs wrote on Oct 27^th, 2020 at 10:15am:


I recommend all of the above. 
You say you go through progressive grits but how exactly are you sanding?
Are you using a sander, backing pad, by hand, etc.

Another vote for Vince's blue discs. They cut longer and faster than any other disc I have used. Maybe not best price, but best value for sure.

As for the mystery scratches, well, even I, after 20 years of turning and thousands of bowls, some times have to go back and sand some more... Good lighting and good glasses, if you need them, are essential. Some one on this forum said "Never take a finished piece from the shop into the house on a sunny day. Sunlight causes scratches." True, but mostly it is because our eyes evolved to see in natural sun light and most shop lighting is not in natural spectrums.

Sand with both your drill and lathe in low speed range. I keep my slow speed drill at half speed, and my lathe at 50 rpm max. Abrasives cut better at slow speeds because they have a chance to dig in and cut, where at high speeds you are spinning your wheels but not going/cutting very fast. I spend more time with grits up to 120 or so, than I do with grits over that. The coarser grits are for smoothing things out and removing light tear out. The higher grits are for removing scratches from the lower grits. If I start at 80, then I go to 100, and 120, and 150, and 180. According to most, I can skip some grits in there, but I think it saves time. If I see a scratch that needs to be sanded out, it is faster to sand out 80 grit scratches with 100 grit than it is with 120 grit, and yes, that includes time spent with the extra grit in the rotation. I try to use different parts of the pad so I can get contrasting sanding scratches/arcs. So, on the outside of the bowl, with one grit I sand with the upper part of the pad, and with the next grit I sand with the lower part of the pad. I wipe the surface down with my hand, and the sanding dust will work into the scratches of the lower grit and high light them. This works better with higher grits than with lower grits... In the inside of the bowl, I tend to sand with the sides of the pad so I get scratches like this ( on one edge, and like this ) with the other side. Generally I sand till I think I have all the scratches out, and then hit it one more time. I do use a firm/hard pad on grits up to 180 or 220. I use a medium pad on grits up to 400. If I go over that, then I use a soft pad.

robo hippy

Woodturning is a three step process .....  turn, sand, then finish.  Mess up on any of the steps and you have a less than desirable product.

When sanding with grits like 80, 100, 120 I am sometimes working a small radius transition zone where it is hard to get to the wood and I am more likely to press harder on the wood.  Which I think is going to leave more prominent deep swirl scratches.

On the outside of bowls I have recently been noticing at about 150 or 220 grit some small circular scratches on the convex surfaces, which I would have expected to be removed with 120 grit.  Probably sanding these areas with just the small center area of the firm pad and so not getting a full eight passes over each spot.  Fortunately these scratches are easy to see in a strong raking light.

On spalted or soft woods, I apply a finish before sanding just to firm up the fibers. Let finish set before sanding.

Glenn J.