What is white powder on component leads?
When tin, a common plating on components, is left exposed to the air, it will oxidize over time, forming tin oxide, a white powder that is insoluble in water.  I believe that if your components are bright tin, the oxide layer will be fairly smooth and not "powdery", which may be the case if you have components plated with matte tin.

The flux in your solder (paste or rosin-core wire) should be able to remove a fair bit of the oxides, but if they are extremely oxidized you may have to add additional flux, or clean the parts somehow.