Cleaning copper before welding is easy and doesn’t require a lot of materials. You must ensure the copper has no paint residue, dirt, debris, oil, or grease on its surface.
These could release toxic fumes when in contact with the high welder temperatures, so you have to get them out of the way.
The simplest way to do so is to use a bronze wire brush and scrub the surface of the copper. You can also use degreasing agents to eliminate potential chemicals and oils. Keep the brush handy and use it even after each deposited weld run.