If you can't drain the water below the defect then you need to buff the screatch by hand. Otherwise you risk getting oil into your water..., eh hum Gulf of Mexico!
not with a pneumatic random orbital buffer
http://www.youtube.com/watch?v=dNFSIvicf5k
anyone have experience with it?
I'd be careful unless using a device specially designed for aquarium/underwater. Pneumatic stuff has oil inside and i spews a minute oil mist as its used, and they need occasional additions of oil as ongoing maintenance.
It's possible the guy with video has a special device, or designed his special, or maybe is using a specialized lubricant that's fish safe to lube the orbital sander as needed.
What size tank is it? There are several large public aquarium maintenance companies that specialize in this type of service. Its very costly but if you are willing to pay then it can be done.
Even if you use a DA sander it will take hours.
Here is a list of equipment you will need. If you don't have any of these items it will cost you around $2000.00 to get started.
-Purchase an air compressor that can produce 90psi at 20 CFM+
-Purchase an DA sander "make sure to clean it in fresh water first to remove oils"
-Purchase micromesh sanding dics from 1500-12000.
Hope this helps.