Rinsing is a good idea. It will also get rid of the old oil.
Smooth the marred backplate face with a file. Just smooth it. there is no need to remove all scoring marks, because they fit the conrod wear pattern.
Do not use sandpaper. Minute corund particles may embed in the aluminum and play havoc with your work.
A slight burnishing operation using the new oil and a smooth hard steel surface will harden the aluminum surface, and prepare it for the next running in. Wipe clean, and apply a fresh coat of oil.
Soak the conrod bigend bearing and ball bearings with oil before you re-assemble.