[Moon] protective coating for soldered joints outdoors

[peter blair]

 I keep on having problems with soldered joint corrosion on the junctions between 141 cable inners ( which are silver plated ) and  my 23cm and 13cm folded dipoles which are made from copper rod.  I cut slots in the ends and thus clamp the wires as well as soldering but I find that the joints are corroding. They are made with resin cored solder, not acid flux.  The joints are out in the weather under protection that keeps direct rain off them. Im looking for some compound to coat them with after soldering. 
  In the absence of other ideas I will  try just clear varnish.
 Any ideas ... this is my tropo system I should add  but these reflectors are usually the place to find a good answer.
 thanks 
73 Peter G3LTF 
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