You do not need to add additional solder. The reason ppl do this is to get new flux on the joint so the solder will flow. You can buy some special flux- I suggest RMA flux for $2-3.
Yes, remove the coating first.
Clean post soldering with alcohol to remove flux.
01 vw A4 TDI
66 Chevy Corsa
68 GMC V6 w/oD