First of all, BFMs do not like to bond to (or flow over) dirt, greases or oxides. The presence of any of these contaminants on the surface of parts to be brazed can literally prevent the BFM from flowing over surfaces on which any of these contaminants are located. Therefore, stop-off compounds, which are blends of metallic-oxides that are packaged in a variety of forms (liquids, pastes, powders, sprays or tapes to name just a few), can be very effective at preventing molten BFM flow into areas protected by the stop-off.
A typical brazing stop-off will be a mixture of metallic-oxide powders (such as aluminum-oxide, titanium oxide, yttrium-oxide, magnesium-oxide, etc.) that is mixed with a liquid carrier solution to form a slurry or suspension that looks similar in consistency and viscosity to thin house paint. It can be applied onto a metal surface by using a small brush, by dipping or by spraying.
Usually a stop-off is applied as a thin, continuous line around critical areas on a metal's surface where BFM must not be allowed to flow. Since BFMs do not like to bond to or flow over oxides on metal surfaces, the presence of an adherent line of stop-off on a metal's surface can effectively tell the BFM: "This far, but no further."
All the readily available metallic-oxide powders used in the manufacture of brazing stop-offs are essentially white in color. Thus, all commercial brazing stop-offs would look alike if no steps were taken to differentiate them in such a way as to make one manufacturer's product look different from the others. This is very effectively achieved by adding different colors to each of the different stop-offs. Today, most commercially available brazing stop-offs are specified by their color, such as pink, red, green, yellow, blue, etc. Few manufacturers use colors already being used by another manufacturer. Thus, if you know the color of the stop-off product you wish to use, it fairly well also defines who the manufacturer of that product is.
Next time we will finish our discussion, looking at cautions for the use of stop-offs.