Although not practical, I have used this method to get a mirror
finish on stainless steel (flat surfaces only) using a Logitech model
PM5 lapping/polishing machine.
I start with 15 micron calcined alumina mixed with water (1 part
alumina: 10 parts water) on a 15" serrated cast iron lapping plate
that rotates at 70 RPM with about 3kg pressure applied to the part.
This step will make the surface planar. It can either take a few
minutes for small flat surfaces, or several hours for large slightly
lumpy surfaces. The tool top speed is 70RPM, faster would certainly
speed up the process, but it would also sling the slurry off the
plate faster wasting the abrasive.
Next, I use 3 micron alumina on the same, but cleaned, cast iron
lapping plate. The speed is 70RPM with 3kg pressure until all of the
15 micron features (not really scratches) are removed. Then I back
off the speed and pressure to about 30RPM and 1kg pressure. Although
dark, the surface should start to mirror (undistorted reflections in
the surface). At this point I adjust the speed, pressure, and slurry
load until there are no visible scratches.
Once I have a scratch free surface, I switch to polishing plate that
has an adhesive backed cloth that I charge with 3 micron cerium oxide
slurry. I run this condition for one hour to remove the most of the
dark mirror look, and then add a colloidal silica drip(product SF1,
0.125 micron). The colloidal silica has a high pH that weakens the
metal bonds, and the abrasive particle wipes the metal away. It’s
chemical mechanical polishing (CMP) at its best, where the cerium
oxide helps keep the mechanical removal ahead of the chemical
removal. If I don’t use the cerium oxide, the colloidal silica etches
too fast, attacking the grain boundary, and leaving an orange peel
surface. I usually run the colloidal silica drip for 30 to 45
minutes. Any longer, and the polishing starts setting up a wave on an
otherwise very flat surface (1 micrometer over 100 mm convex works
best for me!).
As others have said, remove as many scratches as you can, because
they will show up in the polish.
I took some pictures of a 100mm ss disk I did several years ago. It
has a lot of new scratches that the camera doesn’t seem to pick up.
It makes my heart skip every time I take it out to look at it.
Email me off list if you want to see the pictures.