They make underwater polishing equipment (not even close to cheap) for that purpose.

I thought I'd mentioned earlier, maybe I dreamed it, or it's the fatigue & allergy meds, but throw away that SS brush. It's slowly damaging your finish (maybe even causing "sand"). Get a nice high quality nylon brush. SS brushes are too aggressive for nice finishes. Older pools with a bad surface and algae? Go for it. Otherwise no.