To re-do this properly you will need to remove the bezel (metal that holds the clear crystal) and remove the
crystal from the bezel.
The bezel is often removed by undoing a small snap ring underneath. Some indicators have tiny setscrews
on the side. You need to inspect closely to see what the attachment is.
The sad story is if you use solvents or immerse the crystal in cleaning solution, it will often shrink and then 'fall through'
the front of the bezel, rather than properly snapping in place.
My go-to polish up for plastic crystals like this is Simichrome polish, which I think is basically cerium oxide in a binder.
After polishing you need to remove any polish left over on the surface. Again avoid solvents.
Cutting to the chase, if the crystal shrinks badly or simply does not clean up, you can make a new one.
1) find some lexan plastic of the correct thickness.
2) put some fine grained wood in a four jaw chuck and turn a slightly concave surface.
3) screw the lexan to the front and using a heat gun, soften the plastic and let it conform to the concave surface. (lathe NOT turning)
You can encourage the plastic to conform by wiping with a soft rag.
4) using a trepanning tool, cut out the resulting shape at the correct diameter.
Often the first one you make won't be very good. After three or four, you'll get the knack.
