Re: cleaning copper plate prior applying ground
Series of sandpapers is typically fine, for example, for a very rough plate try wet sanding with grit 1000 and then 1600, 2000 and 3200. My copper plate supplier typically ships pretty shiny plates, so I use only 3200 sandpaper and then degrease with Kikkoman (or any similar) soy sauce.
Some car paint/lacquer polishing pastes are also suitable for polishing copper plates. Often there are three grades (coarse, medium and fine) available and the fine is way above 3000 grit and is suitable for very fine polishing. I also recommend having a look at jewellery/goldsmith polishing waxes. The finest ones will give you mirror finish, but will require careful degreasing with dishwashing soap and then with soy sauce.