Some other things that were mentioned were other ways to weatherproof and increase the clean metals for a better solder connection. This is where training, experience and intended application come into effect.
For soldering, yes a light acid does help out alot. If you are in the market for Solder rosin-core helps out. For best results check what tip size and how much heat you are applying. Too much heat can be a bad thing. Please consider researching the correct union for the application since the western union is only for wires of the same type and diameter. I like to use a liquid or gel type flux in addition to the rosin-core solder. it is a light acid and it will etch the surface to allow the solder to bond better. Once complete the solder job should look shiny and have just enough. This is were it gets into training, kinda like welding, or sweating a pipe (aka brazing).
Another weatherproofing item that has been used by auto manufactures is mastic tape. It is the stuff that is used in below ground sealing, like telephone line splices. THIS STUFF IS A MAJOR PITA TO REMOVE. so be careful in using it. The tar like rubber backing in dyna mat is like mastic tape. I hate removing the stuff as it usually results in me cutting myself trying to remove it, and is best to just cut that section out. I do use the stuff but in very few applications.
Does anyone know of a connections thread? is this something that should be made as a sticky. Is this something that would be useful to other members? I know that I am not the only person in the electronics field here. I would like to help out where I can. I have been an Electronics Technician for about 12 years now, I am AD and have been doing the job everywhere from Guam to several locations here in Alaska.