Pounding wire solder flat for this application is something I also do sometimes, but the "pounding" process can coat the surface of the flattened solder with dirt. After the solder is flat, scrape it or sand it until it shines, much as you would clean the solder remaining on the parts.
I have met some techs who save the solder "splatters" that hit the floor for use in this technique.
I have met some techs who save the solder "splatters" that hit the floor for use in this technique.