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"tinning butter" is used on auto body solder. Is there an equivalent for soldering brass instruments? I hate dealing with regular flux burning off as soon as the torch hits and trying to get the solder all the way inside a large joint. It seems that a flux that also "tinned" the brass would facilitate soldering something such as a large bow.

I find a paste flux, such as Rubyfluid, is easier to use than liquid flux.

I agree with roweenie.

Also I tin the joint areas before assembly. If the areas are tinned and clean they will easily draw solder with very little flux.