Core type is important and dependant upon what you are soldering and yes do use flux as mentioned also.

For electirical such as extending and spliceing servo leads, spliceing antennas use only rosin core solder. Using acid core will cause electrical failure at some point.

For piano wire such as solder on pushrod ends or wire landing gear with wrapped connections and cabine struts it is very difficult to use rosin core solder if not impossible for bigger wires. Use only acid core in these application.

Acid core solder is not normally avialible in the thin solder gages so using acid core requires some thing with a bigger tip and heat is required centainly more than say a little 25 amp pencil iron intended only for small wiring.


John