You are hand soldering, presumably using a flux.   All fluxes are made to react with metal oxides on the surface.  Often when they react, they form highly insoluble organo-metallic compounds.  This may be what you are seeing.   If it is, the ideal way to remove them is to use some sort of "saponifier", and then water rinse.

Rudy Sedlak
RD Chemical Company 



      

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