You can buy a soldering flux in paste form the contains the Zinc chloride it is pretty easy to work with. Any store selling plumbing should have it. They have plain and tinning varies it is all lead free any more. I would use plain in order make my joint as pure stay-brite as possible. To make cut acid place about an ounce of muratic (Hydrochloric acid) in a glass container (please wear safety glasses and rubber gloves) usually I use an old ash tray. then I cut two or three 1/4 square pieces of galvanized sheet metal and drop them in the acid they will fiss until the Zinc is eat off the steel. Now you have Zinc Chloride flux. It does two things the most important is it cleans the second is aids in the binding and flow of solder molecules to the base metals. just apply this to where you want the solder to go and only where you want it to go.
Good luck
