When I finish a stone sculpture I use wet/dry sandpaper to polish the stone. This works very nicely with alabaster, chlorite, and wonderstone (pyrophyllite). I don’t do marble, so I can’t tell you much about how to polish marble.
But when it comes to polishing alabaster I’m a bit of an old hand. I’ve polished about 100 different sculptures to date — some small, small large.
After taking the sculpture as far as I intend with rasp, I start in using 120 grit sandpaper. I used to use 3M 150 wet/dry. Boy that was a sweet sandpaper. Can’t get it much anymore — leastwise not the same quality as the old black stuff. Too bad I didn’t stock up more.
The job of the 120 grit sandpaper is to remove all, All, ALL scratches left by the rasping process. If I get anxious and move on to 220 grit (the next sandpaper in the process) before all of the rasp marks are gone, then those rasp marks will be there at the end. This 120 to 220 barrier is an important one.
The sizes of sandpaper I use are 120, 220, 320, 400, 600, 1500. If I move from 220 to 320 a little early it is not such a big deal. The 320 can (with a little extra effort) do the job of the 220. But the 220 just can’t manage the job that 120 is designed for. Admittedly it is important to finish with each stage of sandpaper before moving on to the next. Even so, the 120 to 220 is extremely important.
Back in the ol’ day — long before I learned this lesson the hard way — I would be working with 600 grit and have a very smooth sculpture with a very well defined rasp mark that would just not come out. I tried backing down to 400 to remove it — no go. I tried backing down to 320 to remove it — no go. I tried backing down to 220 — no go. I had to go back to the 120 to remove the mark. Then, of course, I had to re-polish with each of the intermediate grits back up to 600.
You’d think I would have learned my lesson after the first time — lol. Well, at least I learned it after the second time. Since then I am very careful to make sure that all rasp marks are gone before moving from 120 to 220.