In addition to the stop-offs mentioned last time, please note that milk of magnesia, readily and cheaply available from many drug and food stores, is actually a decent stop-off that may prove effective for many general-purpose needs. So, do some experimentation and find out what kind of stop-off works best for you.  

But – VERY IMPORTANT – don't let the easy availability of stop-offs lull you into carelessness in the brazing process, as discussed below!  

CAUTIONS:  

1. Overuse – Because of the effectiveness of stop-offs, many brazing shops tend to lose concern about the amount of BFM they apply to parts, merely saying: "Oh, don't worry about how much BFM is used, just put on more stop-off." That is a VERY bad idea! Stop-off should never be used as an excuse for allowing the use of too much BFM. Unfortunately, I see too many shops that overuse brazing stop-offs rather than training their people to back off from such overuse. Instead, they should only use the correct amount of BFM that is needed to make a good brazement and only small amounts of stop-off if it is absolutely necessary.  

2. Correcting stop-off "mistakes" – It is not uncommon for persons applying the stop-off to make a mistake and accidentally apply it onto a surface that needs to be brazed. I've heard some folks very softly say: "Uh, oh..." as they're putting stop-off onto parts. When I look in their direction to see what they just did, I see them taking a cloth and dipping it into a solution such as acetone or alcohol with the intent of wiping the stop-off from the surfaces where it is not supposed to be. THIS DOES NOT WORK!  

Please understand that the following saying is very true: "Once stopped-off, always stopped-off." You cannot merely wipe the stop-off from the surface and expect that surface will then become brazeable again. That will not occur. Residues from the stop-off are still there, and the BFM will avoid wetting that surface.

The only way to effectively remove stop-off from surfaces is to either thoroughly ultrasonically clean that surface or machine off the contaminated top surface layer. Better yet, find ways to eliminate the use of stop-off altogether. Too many shops plan on the use of stop-offs on too many of their parts, thinking that it is always wise to do so. No, it is not.

CONCLUSION:

Brazing stop-off can be a helpful tool for a brazing shop, but it should be used sparingly and only when absolutely needed. Otherwise, overuse of stop-offs can actually result in more-than-necessary scrap and rework of parts.