Solder & Flux: Mark Nelson Shares the Nitty Gritty
Barrier fluxes would be fluxes such as Cupronil, Stop-Ox, or Firff . There are spray-on fluxes available, but a good homemade version is a 60/40 mix of boric acid and de-natured alcohol. My-T-Flux and Handy Flux are two of my favorite examples of flow fluxes. I use Handy Flux mostly for silver and My-T-Flux for gold.