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With regard to keying. You are going to have to first clean the entire scratch with either detergent or wax and grease remover. Next, apply primer to any bare metal areas. Now start with the basecoat and follow with the clearcoat. When I have fixed key scratches, I use rubbing compound instead of sand paper to keep the primer, paint and the clearcoat smooth. Practice on one small area first and if it doesn't look good, wash it off with automotive lacquer thinner and try again. After a few attempts, you will really improve. I am assuming you have never fixed a scratch before.
How good can it look? If you have a solid color (non-metallic), you can make the scratch go away completely. If you have a metallic color, and most vehicles are, depending on the color, you can sure make it look much better, but it may be noticeable still on close inspection.